Fixes #17 Updated to use requests 1.x

Requests 1.x no longer has an error attribute in Response objects,
and Response objects now expect a `reason` attribute in the
raise_for_status() method when building the error message. This was
causing errors in tests as well as any requests.

requirements.txt now specifies requests >= 1.0.0

tests/utils has had MockResponses updated to reflect the API change,
as well test_properties_copied() in tests/test_resource.

Tests have been run in requests 1.0.0 and 1.1.0