Make PyCrypto optional if you're not using any of the RSASSA-PKCS1-v1_5 algorithms

The latest update of PyJWT includes support for RSASSA-PKCS1-v1_5 algorithms. Installing PyJWT does not install its newest dependency of PyCrypto. This PR is a proposal to keep the installation PyCrypto optional if you're not even using those algorithms, leaving PyJWT self contained and dependency-less.