Crux

**Changes:**

- Add `Crux.get_resource()` to get resources by ID.

**BREAKING CHANGES:**

- Change `Dataset.add_permission()` to set dataset permissions, add `Dataset.add_permission_to_resources()` for bulk resource permission updates.


**Changes:**

- Reuse request session to allow TCP connection to be reused.
- Add `Crux.close()` to close session, for uses that create many `Crux` clients.
- Support proxies signed GCS uploads/downloads.
- enum34 no longer vendored.

**Changes:**

- Fix setup.py to support Python 2.7

**Changes:**

- Add support for direct from GCS downloads
- Add pagination to label search
- Add logging functionality
- Fix bug that prevented small files from being downloaded directly from GCS

**Breaking changes:**

- Some required args in download/upload methods were renamed to improve accuracy of arg names

**BREAKING CHANGES:**

- Many internal implementation details have been made private by adding
an underscore to the module names.
- `content_type` arg changed to `media_type` to match API naming.
- Labels changed to a dictionary to allow simple k/v lookups.