Enable twofish as an optional dependency

[blim747]

Problem:

As of now pyjks requires the install of twofish, even though according to the README twofish is only required to read UBER keystores. twofish is a package that has no commits since 2013 and is not compatible with Python 3.3+. Installing twofish should be optional.

Solution:

Enable twofish to be installed as an extras_require.

Testing:

I was able to build with Python 2 and Python 3 without issues, with and without twofish. tox tests pass for all Python versions there are tests for.

Notes:

• See Move off requirements.txt and enable twofish as an optional install #68 for the previous draft version of this PR with a polluted commit history. The previous change included various changes to setup.cfg and requirements.txt that are not required to enable twofish to be installed optionally.
• Question: Can we increment the version number as part of this change?

[Warkdev]

Any chance to see this merged and released ? :)