Add X-RateLimit-Reset to response headers

As per https://developer.github.com/v3/#rate-limiting the
X-RateLimit-Reset header is included in every response and is the time
in UTC epoch seconds when the current ratelimit window resets.
Regen API with X-RateLimit-Reset response header
Add /rate_limit API

Retrieves the current rate limit status for both core and search.

See https://developer.github.com/v3/rate_limit/
Regen API to include /rate_limit
Add test for misc.rateLimit

Generated, then modified
Merge pull request #164 from strongloop/rate-limit

Rate limit API and enhancements
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