Added support for If-Modified-Since & Last-Updated headers

This pull-request aims at offering support for If-Modified-Since & Last-Updated headers to thumbor. This would provide a better communication with CDNs such as Akamai or CloudFront which will avoid asking the media to often by asking if it has been modified before.

This PR should be BC (I've added an exception catch if the result storage doesn't implement the required method, which logs a warning).