Renamed/split HTTPSeekRequest & HTTPResponseElement classes

Per feedback on gnome bugzilla 710628, split classes defined in
 rygel-http-byte-seek.vala and rygel-http-time-seek.vala into their
 own files to maintain the one-class-to-one-file Rygel convention.

Did a couple tests with TimeSeekRange, PlaySpeed, and both in the same request.