Tsrc

* *Breaking change*: Add support for groups (30). Reported by arnaudgelas

tsrc push --assignee`: fix when there are more than 50 GitLab users (25). Reported by arnaudgelas

* Split user interface functionality into its own project: [python-cli-ui](https://github.com/TankerApp/python-cli-ui).

* Add `--quiet` and `--color` global options.

Bug fix release.

* `tsrc init`: Fix crash when a repository is empty (17). Reported by nicolasbrechet
* `tsrc push`: Fix rude message when credentials are missing (20). Reported by cgestes

Packaging fixes.

* Support for specifying custom branches in the manifest
* Support for specifying fixed refs (tags or hashes) in the manifest

New syntax is:

yaml
repos:
- src: foo
url: gitgitlab.com:proj/foo
branch: next

- src: bar
url: gitgitlab.com:proj/bar
branch: master
ref: v0.1


Note that `branch` is still required.

* You can now skip the `dest` part of the `copy` section if `src` and `dest` are
equal:

yaml
copy:
- src:foo

same thing as
copy:
- src: foo
dest: foo