Git-up

- Prior to v1.1.0, `PyGitUp` tried to guess the upstream branch for a
local branch by looking for a branch on any remote with the same
name. With v1.1.0, `PyGitUp` stops guessing and uses the upstream
branch config instead.

This by the way fixes issue 6 (`git up` doesn't work with
local only branches).

**Note:** This change may break setups, where a local branch
accidently has the same name as a remote branch without any tracking
information set. Prior to v1.1.0, `git up` would still fetch and
rebase from the remote branch. If you run into troubles with such a
setup, setting tracking information using
`git branch -u <remote>/<remote branch> <local branch>` should help.
- 3rd party dependencies have been updated.
- Allows to run `git up --version` from non-git dirs, too.

Finally `PyGitUp` reaches 1.0.0. You can consider it stable now :)
- Added a comprehensive test suite, now with a coverage of about 90%.
- Lots of code cleanup.
- Added option `-h` to display a help screen (`--help` **won't** work, because
`git` catchs this option and handles it before `PyGitUp` can do).
- Added option `--version` to show, what version of `PyGitUp` is running.
Also checks for updates (can be disabled, see README).
- Added option `--quiet` to be quiet and only display error messages.