Json encodeable jwk dict

[blag]

Closes #127.
Closes #137.
Closes #139.

[codecov]

Codecov Report

Merging #165 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #165   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          14       14           
  Lines        1062     1062           
=======================================
  Hits         1032     1032           
  Misses         30       30

Impacted Files	Coverage Δ
jose/backends/ecdsa_backend.py	98.66% <ø> (ø)	⬆️
jose/backends/cryptography_backend.py	98.11% <ø> (ø)	⬆️
jose/backends/rsa_backend.py	96.5% <ø> (ø)	⬆️
jose/backends/pycrypto_backend.py	95.76% <ø> (ø)	⬆️
jose/jwk.py	95.06% <ø> (ø)	⬆️

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