Support conditional API requests

[mikemorris]

Using Last-Modified``If-Modified-Since or ETag``If-None-Match headers http://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.

[clayreimann]

@mikemorris How would you expect such a feature to work? Would we cache all of the requests and then return the previous value if we get 304 Not Modified? Or would you expect to pass a last-modified or ETag as a parameter to the function?

[mikemorris]

Decision to cache would likely be made by an implementing app, not here, but yes, I would expect to pass an Etag or last modified date into the function.