Move to Python 3.4+ only

[tleonhardt]

cmd2 now supports Python 3.4+ only for versions starting with 0.9.0. This PR starts removing support for Python 2.7 and begins cleaning up things which are no longer needed.

Changes include:

• No longer run Python 2.7 unit tests on either TravisCI or AppVeyor
• On AppVeyor, Windows code coverage analysis is now done for Python 3.6 instead of 2.7
• 3rd party mock module is now only used for tests on Python 3.5
• Cleaned up most references to Python 2.7 in the docs
• Removed all dependency on the six module
• Removed conditional code branches which were for Python 2.7
• Started using super() to call base class dunder init

This closes #260

[codecov]

Codecov Report

Merging #354 into master will increase coverage by 0.05%.
The diff coverage is 73.07%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   89.26%   89.31%   +0.05%     
==========================================
  Files           1        1              
  Lines        1909     1872      -37     
==========================================
- Hits         1704     1672      -32     
+ Misses        205      200       -5

Impacted Files	Coverage Δ
cmd2.py	89.31% <73.07%> (+0.05%)	⬆️

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[tleonhardt]

I'm going to go ahead and merge this. We can clean up and/or change in future PRs if need be. But future work will depend on this, so I'm going to merge it in.