support for If-Modified-Since conditional requests

* support for all subtypes of BaseEntity
* currently no support for collections
removed obsolete method
Merge pull request #201 from autermann/feature/if-modified-since

Support for If-Modified-Since conditional requests
Merge remote-tracking branch 'upstream/dev' into feature/range-based-pagination

* upstream/dev:
  removed obsolete method
  support for If-Modified-Since conditional requests

Conflicts:
	rest/src/main/java/org/envirocar/server/rest/guice/JerseyModule.java