added test case to repeat error in #13

checks to make sure the server does not crash and instead returns a 400 when invalid timestamps are provided
added error handling in API to fix issue #13

catches the ValueError thrown in resources.py if an invalid timestamp is passed as a parameter and raises a BadRequestException, allowing it to propogate up into api.py where it is returned as a BadRequest (400) to the client with a useful status message.
added test case which tests to see if JSON was returned if no Accept header was provided
If no ACCEPT header is provided in the HTTP Request, application/json will be used as the default
added test cases to replicate #11 (server crashes if no JSON data in a POST)
caught value error thrown if illegal json was provided in post requests, fixing issue #11

instead of crashing server, the bad request now triggers a 400 BadRequest response
PEP8 formatting
now get_response in tests will parse based on the response's content type
Merge branch 'master' into pr/17
Merge branch 'master' into pr/18
PEP8 fixes