Assigner 2.0.0

There's a lot in this release! Most exciting is the commit and push commands which allow you to modify student repositories, for example to add a gradesheet to submitted assignments.

There are also a handful of papercuts that have been fixed, a few bugs, and some under the hood refactoring to make future modifications easier. Enjoy!

Changes from 1.2.0:

• Added 'commit' command to commit changes to student repos
• Added 'push' command to push commits to student repos
• Retry clones that fail with 'Connection reset by peer' using exponential backoff
• Offer to create GitLab group in assigner init
• Version configuration; automatically upgrade old configs to latest version
• Make GitLab backend configuration generic
• Tidy up progress bars with logging output
• Test all supported python versions (3.4-3.7) automatically
• Added BackendBase, RepoBase, StudentRepoBase, and TemplateRepoBase.
• Base Repos are now called Template Repos.
• Separated Gitlab backend from most of the code. Added Base implementations for Gitlab.
• Added requires_backend_and_config decorator with backend config option to load the desired backend.
• Renamed config_context decorator to requires_config.
• pyflakes 1.6.0 upgrade for type annotations fixes.
• Improved canvas error handling.