Unhandled exception in case of bad request fixed.

If we make request with wrong token we get unhandled exception because
X-Ratelimit-Remaining and X-Ratelimit-Limin headers are not set in
response.
Added checking for X-Ratelimit-Remaining and X-Ratelimit-Limit after
getting response from server.
Merge remote-tracking branch 'upstream/master'
Merge pull request #31 from drussilla/master

Unhandled exception fixed
Merge pull request #1 from InstaSharp/master

Pull new commits