Update language support tests and docs

[JordonPhillips]

Adds python 3.6 the tox configuration and adds python 3.5 & 3.6 to
the language classifiers and travis configuration.

Resolves #164

cc @kyleknap @jamesls @dstufft @stealthycoin

[JordonPhillips]

Looks like I'm going to have to mess around with this a bit to get it to work beyond python 3.4

[dstufft]

Oh, it's because you don't have the python: lines in .travis.yml.

[dstufft]

These things: https://github.com/boto/boto3/blob/develop/.travis.yml#L2-L8, because Travis only preloads some Pythons not all of them for a variety of reasons, and they download the rest on demand when the language selector picks it. You can do something like https://github.com/pypa/pip/blob/master/.travis.yml#L4-L33 to get the TOXENV variable set along with the python version, or you need a script that will set it correctly.

[JordonPhillips]

Looks like that did the trick!