Adding .py file to make running work out of the box on windows

Attempting to provide a better solution to nose-devs/nose#487 .

**Goal:** provide out-of the box runner on windows

**Weaknesses of this solution:** 
* code duplication with `bin/nosetests` (4 LOCs)
* relies on extension association between ".py" files and the python interpreter

**Strengths of this solution:**
* KISS
* works with cmd.exe and any other runner, as long as ".py" files are associated with the python interpreter
* python solution => cross-platform (linux, MsysGit, <untested others...> )