Add Last-Modified header to Art API handler

Fix for Issue #64.

I am currently experiencing problems with my "last modified" times on all WaveBox items.  I do not know if this is a Linux issue, a filesystem issue (XFS), or what else it could be.  Will continue to investigate.  For now, I've added a sanity check, which will not add the Last-Modified header if the UNIX timestamp is greater than Int32.MaxValue (See Year 2038 problem).