Support conditional API requests

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

Start by locating the repository's API request entry point and review how requests and response headers are currently handled. Use the linked GitHub conditional-requests documentation to determine how Last-Modified/If-Modified-Since and ETag/If-None-Match should be supported, then verify that conditional API requests behave accordingly.

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