Support X509 certificates with cryptography

[mpdavis]

No description provided.

[codecov]

Codecov Report

Merging #107 into master will increase coverage by 0.38%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   94.78%   95.16%   +0.38%     
==========================================
  Files          13       13              
  Lines         997     1159     +162     
==========================================
+ Hits          945     1103     +158     
- Misses         52       56       +4

Impacted Files	Coverage Δ
jose/backends/cryptography_backend.py	96.89% <77.77%> (-0.97%)	⬇️
jose/jwt.py	100% <0%> (ø)	⬆️
jose/backends/pycrypto_backend.py	96.59% <0%> (+0.23%)	⬆️
jose/jws.py	96.98% <0%> (+0.34%)	⬆️
jose/backends/rsa_backend.py	97.89% <0%> (+0.69%)	⬆️

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