Support conditional API requests

[mikemorris]

Using Last-Modified``If-Modified-Since or ETag``If-None-Match headers https://developer.github.com/v3/#conditional-requests

[w35l3y]

👍

I think is it related to #45

[aendra-rininsland]

Right you are, @w35l3y. Closing as duplicate of #45.

[aendra-rininsland]

This is actually a separate issue. Reopening.