Implementing If-None-Match and If-Modified-Since

Updating the GET implementation so that the If-None-Match and If-Modified-Since headers are handled.  This is especially useful for clients that want to cache resources, provided they save the ETag or Last-Modified headers in preceding GET requests.

Note: Sorry about the back and forth Curtis.  I misunderstood what you were looking for in the last submission.  This has only the changes associated with this update and none of the merges.  

Thanks!