Merge pull request #36 from kellrott/master

Fixing spelling mistake that breaks provenance code
Merge pull request #37 from kellrott/master

Fixing id_of to return value
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
early attempt at entity object
Entity class
add constructors for entity subclasses
move date functions to utils and add another test for entity subclasses
create one Synapse instance for multiple tests
fix id_of to raise exceptions
change names of unix epoch time functions
improved entity object
try an alternate implementation of Entity without changing the behavior of the dot notation
automate construction of entity_type_to_class map
removed entity2
fixed __repr__ method
improved entity.__repr__ and .__str__, added createDataEntity to integration test