setup: make Python version classifiers more generic

We now support a wide range of 2.x and 3.x versions, so I don't think
we need to list the specific versions any longer.

Furthermore, websites such as http://python3wos.appspot.com/ monitor
the 'Programming Language :: Python :: 3' tag to report on the support
for Python 3.
readme: add build status icon
Merge pull request #91 from mgeisler/misc-doc-fixes

Small doc fixes