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
Enforcing a common interface for `sign_*()` methods...

In order to support adding custom signature methods, the current signature methods -- HMAC-SHA1, RSA-SHA1, and PLAINTEXT -- need to be implemented with a common interface. In a previous attempt, I tried changing those functions directly, but there are too many dependencies on their current signatures. By shimming them instead with these thin wrappers, I can provide the common interface I need without breaking everything else in the library.
Allowing custom signature methods to be registered and used...

The "PIZZA" signature method signs all requests with the string "PIZZA" as a trivial example of a custom signing method.
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 #239 from aspiredu/master

Add support for custom signature methods to `oauth1.Client`
Merge pull request #237 from jturmel/add_crypto_token_generator

Add crypto token capability
Merge pull request #240 from asteinlein/master

Support for scopes being arbitrary objects
Updated AUTHORS with Josh Turmel

* Coincides with signed token change that was accepted recently
Fixed a typo.
Unshadowed a test.

There were two test methods named `test_create_authorization_response`
in `tests.oauth1.rfc5849.endpoints.test_authorization.ResourceEndpointTest`,
so second one was shadowing the first.

Also renamed the test case.
Move docstrings above `from __future__` imports.

Docstrings are considered as such by python only if they are the first
statement in the file/block.  Even if they are preceded by a
`from __future__` import they are interpreted as just a string.
Removed mention of django-oauth-toolkit from oauth1 server docs.

django-oauth-tools doesn't seem to support oauth1.
Merge pull request #243 from jturmel/patch-1

Updated AUTHORS with Josh Turmel
Merge pull request #244 from x746e/master

Unshadowed a test
Merge pull request #245 from x746e/moved-docstrings-to-the-top

Move docstrings above `from __future__` imports.