Support multiple release "tracks"

Bloom should support multiple release configurations per bloom repository, to enable better support for releasing the same library for multiple versions of ROS simultaneously.  In order to support this bloom will need:

- [x] Refactor `git-bloom-config` to support multiple (more detailed) configurations per repository
- [x] Refactor `git-bloom-release` to take care of new configuration features and to support tracks
- [x] Refactor `git-bloom-import-upstream` to be much simpler (takes only a path to a tarball)
- [x] Add `bloom-export-upstream` which takes upstream info and outputs a tarball
- [x] Default to `rosrelease` generator rather than `release` generator for new `release/rosdistro/name/version` convention

All of these tasks are required for bloom to properly support multiple tracks.