bewit's URL safe encoding should not = pad bewits

[jhford]

The bewits generated in mohawk.bewit are returned having padding = characters in them. These equals padding characters cause the bewit to be not url safe. We should .rstrip('=') the bewits generated

[kumar303]

From chatting about this on IRC it sounded like the equal sign was causing errors in node hawk so we should definitely strip it in mohawk. Thanks.