Abstract file downloading out of Repository.archive()
Drop test fixture for old download API
Add release and asset test fixtures
Add Asset#download and #download_url
Add Asset class to the repos documentation

Also add the Release class to the section index.
Convert _stream_response_to_file to a utility function
Move tests for utils module
Add tests around stream_response_to_file
Add more assertions around the mocked file
Remove extra new-line
Add Asset#download integration test
Alphabetize Asset methods
Add integration test for Asset#edit

Caught a bug in the code which is now fixed
Merge branch 'pr/219' into develop