Add a high level bewit interface and docs

[kumar303]

I think we need a higher level interface for bewit (signed URLs) and some good docs. I can work on this. The core functions were added in #13

[kumar303]

@edmorley btw, I pushed the undocumented bewit support to PyPI so you can use it more easily. http://mohawk.readthedocs.org/en/latest/#changelog

[edmorley]

@edmorley btw, I pushed the undocumented bewit support to PyPI so you can use it more easily. http://mohawk.readthedocs.org/en/latest/#changelog

Thank you :-)

CC @catlee @jhford

[kumar303]

Thanks again for implementing bewit!

I documented the low level interface https://mohawk.readthedocs.io/en/latest/usage.html#generating-protected-urls

I still think the interface is a bit awkward so I'll leave this issue open. Here are ideas for improvement:

• The caller shouldn't need to craft a Resource object with an empty nonce.
• A higher level function should also accept an expiration time in seconds rather than require a complete timestamp.
• The higher level function could also generate the final URL itself.
• All exceptions should derive from HawkFail, see Sending a garbage value to check_bewit should raise HawkFail #54
• I'd rather see check_bewit fail only by raising an exception, instead of returning booleans. That would make the calling code easier.