Adds the releases api

NOTE: [Upload release asset](https://developer.github.com/v3/repos/releases/#upload-a-release-asset) is not implemented.

It's unclear how you'd like upload implemeted. It uses a different URL which means some how
the hosts address needs to change for this upload. It also has to send a potentially large
file meaning the file should probably be streamed, rather then passing the entire binary
blob to the function call.
add tests for the release API

note: I can't test editing and deleting an asset until I can create/upload
an asset. The current node-github design doesn't seem to support that
use case. Any thoughts on the best way forward are welcome.
Update README to show releases api coverage percent
Merge pull request #161 from greggman/add-releases-api

Adds the releases api