BF: make windows script wrappers more robust

The script wrappers were hard-coding the python path at the time of the
creation of the binary such as an exe or an egg.  This path is likely to be
wrong after the binary is installed somewhere.  Use some minor bat file
programming to pull the Python command out of the script directly, rather like
the ``cli.exe`` trick used by setuptools.
RF: don't use setuptools to create the .exe binary

We don't need setuptools for bdist_wininst
BF: use shell=True for windows script tests

The default shell=False for windows meant that we weren't picking up
surrounding changes to the PATH, and so not finding scripts installed into a
virtualenv.
DOC: comment in windows bat, rewrite docstring

Comment to help reader of windows bat wrapper.  Point readers to
discussion of installation and bat files.
Merge pull request #257 from matthew-brett/windows-script-install

Make Windows script install more robust

We were getting errors testing the scripts on windows of form:

======================================================================
ERROR: nipy.tests.test_scripts.test_nipy_3_4d
----------------------------------------------------------------------
  ...
WindowsError: [Error 2] The system cannot find the file specified

This turned out to be because I hadn't thought out all the ways the scripts could be installed, in this case, into a virtualenv. This PR makes a more robust wrapper for the scripts, and changes the script running to pick up e.g virtualenv paths.

I documented this at http://matthew-brett.github.com/pydagogue/installing_scripts.html