Continue work on Releases API

- Add delete/edit methods
Rename releases.py
Add Release object to docs
Bump version to avoid confusion for docs
Typo: suppoed -> supposed
DRY + Release#iter_assets
Add Asset class. Flesh out Release#iter_assets
Add Release#asset to retrieve a single asset
Use self._delete since it exists.
Add Release#upload_asset to create asset files
Add Asset#edit. Fix Release#upload_asset

- Fix Release#edit
Add possibly useful helper
Start writing some unit tests
Add repository tests

Devise testing scheme with mocks on the unit level
Attempt to please Travis
Add Repository#iter_releases

Also add integration tests for it
Remove kruft
Add Repository#release

- Add integration tests as well
Forgot the cassette
Start Repository#create_release