Django-arcutils

In progress...

- Added DRF serializer field type: `LocalDateTimeField`. This converts
the time to local time before serializing it to a string so that it
will read as expected when rendered in a template.
- Changed default ordering of `AuditLog` records so that the most
recently added log records come first. Wanting to see the most recent
changes first is probably the most common usage scenario.
- Added `AuditLog.related_changes` property to make it easy to check
if a log record has any related changes.
- Upgraded `certifi` 2017.4.17 to 2017.7.27.1.

- Upgraded django-local-settings 1.0a6 to 1.0a7. The latter doesn't
choke on tuple settings like the former did.
- Enhanced the `UpTime` setting, in particular so that `str` and `repr`
return useful representations.

Made a couple tweaks to the `masquerade` app:

- Made the `is_masquerading` tempate tag more robust by immediately
returning `False` when masquerading is disabled.
- For consistency, always register masquerade routes. This ensures URL
generation always works. When masquerading is disabled, all views
explicitly return a 404 response.

- Template directories are no longer specified in settings. In a typical
scenario, this isn't needed since template directories from installed
apps are already included/searched.
- Removed Logstash logging config and Stashward dependency since we're
not using Logstash (and actually never really did).
- Removed PSU CA certificate since it was only used by Stashward.
- The following settings are no longer added by `init_settings()`:
- `ARCUTILS_PACKAGE_DIR`
- `PACKAGE_DIR`
- `ROOT_DIR` (replaced by `CWD`)
- The following settings are now added by `init_settings()`:
- `CWD`: The current working directory (replaces `ROOT_DIR`)
- `DISTRIBUTION`: The Python distribution; defaults to the `PACKAGE`
setting
- The `DISTRIBUTION` setting is now used to get the Python distribution
instead of `PACKAGE` (used to set the `VERSION` setting).
- Simplified innards of `init_settings()`.
- Added `coverage` dependency.
- Replaced `runtests.py` with a RunCommands command: `run test`. The new
command has options to easily get a coverage report and do linting.
- Added `install` command from ARCTasks.
- `migrations` directories are now ignored when linting via `flake8`.
- Improved `tox` config: added tox-specific pip requirements file.
- Cleaned up `requirements.txt`.

Major Changes

- Added `auditor` app. This is used to log changes to specified model
fields. Should be considered a work in progress for now.

Minor Changes

- Added custom 400 and 500 error views. These pass the request context
to the template renderer so that templates can be rendered correctly.
- Removed global `ARCUTILS_PACKAGE_DIR` from `settings`. The ARCUtils
package directory is now retrieved only when `init_settings()` is
called.
- Added additional default settings in `init_settings()`:
- `UP_TIME`: an object with a `current` property that returns the
current uptime (by subtracting the current time from the existing
`START_TIME` setting)
- `VERSION`: the project version retrieved via `pkg_resources`

Other Changes

- Upgraded djangorestframework 3.6.2 => 3.6.3
- Upgraded ldap3 2.2.2 => 2.2.4
- Upgraded raven 6.0.0 => 6.1.0
- Fixed double-printing of `end` in color printer
- Replaced deprecated `log.warn()` with `log.warning()`