Switch to cryptography for RSA and ECDSA algorithms

Currently, pyjwt uses `PyCrypto` (last release 2013-10) and `ecdsa` in order to support RSA and ECDSA algorithms. Instead of dealing with two dependencies, I propose switching to using the [cryptography](cryptography.io) library for the following reasons:
* Better performance (RSA performance is [30% faster](https://bitbucket.org/markadams/py-crypto-speed-test) than in `PyCrypto`
* More trusted codebase. The `cryptography` library uses OpenSSL for most of its cryptographic functions which is trusted, very familiar, and well maintained.
* Single library for both RSA and ecdsa support
* `cryptography` is more actively maintained. (`PyCrypto` hasn't seen any updates in over a year)

The attached commits swap out PyCrypto and ecdsa for cryptography. There are no public API changes to pyjwt. All existing test cases pass without issue.