Investigate on providing alternative Python implementations

[damienflament]

Actually, only the C Python implementation (denoted as reference implementation) is provided by travis-python.

It would be interesting to provide alternative implementations, like:

• Active Python
• Anaconda
• MiniConda
• IronPython
• Jython
• MicroPython
• Pypy
• Pyston
• Stackless

That list is extracted from python-build definitions. Some implementations may not be available through Chocolatey.

To ease the use of travis-python through the provided shared configuration snippet, depending only on the PYTHON environment variable might be necessary. To do so, expecting the this variable to contain a string matching the format [<implementation>-]<version> might be the more elegant solution where the implementation is optional and means python if omitted.