Allows download of external fileEntity urls by checking url location.
Adds support for externalFileEntities, and some more verbose error messages.
Adds REST functions.
   -Adds the REST CRUD functions and moves requests calls to there.
   -Adds error output (could be even more informative)
   -Changes behavior of 404's to return none instead of raising HTTPException.
Changes dependency of requests to include Chris B's pull request.
Merge pull request #38 from larsssono/master

Fixes external URL download for fileEntities and creation of externalFileEntities