Add crypto token capability

* Add a method to generate crypto tokens for use as Bearer tokens
* Add a method to verify an incoming crypto token and unpack the header and
  claims
* Uses the PyJWT library

This is not JWT token support, merely a way to generate Bearer tokens that
won't have to be stored in a database but can self-validate and store
additional information that you see fit.
Use the to_unicode helper
Change how arbitrary claims are set on crypto token
Require pycrypto and pyjwt
Move setup.py requirements

* Moved requires for cryptotoken changes so that you don't have to install
  the dependencies if you aren't going to use cryptotokens
Changes per PR comments

* Rename methods from crypto to signed
* Since generating a signed token to be used as a refresh token could be a bit
  overkill in terms of length/storage, allow setting of refresh token generator
  separately, or it defaults to whatever the other token generator was set to
* Simplify tests per PR comments
Add pyjwt to tox.ini
Merge pull request #237 from jturmel/add_crypto_token_generator

Add crypto token capability