New compliance hook and error handling

[jamielennox]

• Add a new compliance hook for token_request

Allow us to modify the request that is sent when a new token is requested. This helps in situations where the standard format is not accepted.

• Use requests-mock for testing

requests-mock is already a dependency for some tests but extend it to the more core tests.

• Throw an exception on a bad http return code

Throw an error when you get a non 2XX status code because at that point we are confident there's not going to be a token in the body.

[coveralls]

Coverage increased (+0.3%) to 88.115% when pulling 94bec1b7b2bb0b927abab3315483f45c7ff06000 on jamielennox:compliance-request into 91298c3a2fc7b461862b4273da2f69d43eeb2346 on requests:master.

[jamielennox]

Sorry for how these are combined, my biggest rant about github is not being able to do them as dependencies, but the code is intertwined so I can't completely separate them.

I recommend thinking of it as the 3 PRs itemized in the PR body with the first 2 part of the compliance hook.

If parts of this are an easier merge than other i'm more than willing to split it up in whatever way is most acceptable.