Implement optional backends for python-jose as setup.py extras.

[zejn]

This allows installing running pip install python-jose[cryptography] to install with cryptography. Supported backends libraries are cryptography, pycrypto and cryptodome.

[mpdavis]

I like this approach much better.

Just a couple of issues:

1. It appears to have broken our elliptic curve cryptography backend.
2. Is there a way to determine a default extra dependency? From a quick search it doesn't appear that there is, which is unfortunate. I am always amazed that the amount of people that don't pin their dependencies.
3. We will want to update the docs to reflect this change. This doesn't have to happen in this PR, this is mostly just a reminder to myself.

I will be cutting a 2.0.0 after this lands to try to affect as few people as possible.

[zejn]

1. Not quite, the breakage comes from cryptography 1.9, which was fixed in PR Fix for just released cryptography 1.9. #56, but this pull request's CI was run before that PR was merged. Not sure how to re-run the CI without another commit.
2. I'm implementing a backend using pure python rsa module -- https://pypi.python.org/pypi/rsa/3.4.2 -- so you get basic functionality out of the box, albeit at a bit of a performance hit. I have it working mostly, only having issue with supporting appropriate PEM output.
   I'm not sure how one would go about implementing a "default" dependency. This would be possible if python packaging would have a "provides" field (e.g. like an apache2 deb package has "Provides: httpd"), but I'm not sure that even exists in python packaging, especially since the packages are provided by the upstream developers themselves and provides is a more of an ecosystem integration feature.
3. Indeed.

[codecov-io]

Codecov Report

Merging #57 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   95.64%   95.51%   -0.14%     
==========================================
  Files          12       12              
  Lines         734      735       +1     
==========================================
  Hits          702      702              
- Misses         32       33       +1

Impacted Files	Coverage Δ
jose/backends/pycrypto_backend.py	95.23% <0%> (-1.2%)	⬇️
jose/backends/cryptography_backend.py	100% <0%> (ø)	⬆️

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[GautamGupta]

@mpdavis When do you plan to be able to cut a 2.0?

@zejn Is it possible for you to amend your commit and force push for the tests to re-run? edit: I think it's actually complaining on coverage and not failing tests.