Support multiple Pythons in the same working tree

fixed dev.py on windows

This way you can run the tests against several python versions by doing

  make PYTHON_VER=2.6 build test
  make PYTHON_VER=2.7 build test
  make PYTHON_VER=3.2 build test

repeatedly, without having to re-download and recompile the respective
clean Python interpreters.

removing debugger, adding comments

Fix nondeterministic buildout.txt test failure

If the test happened to write to mkdir.py twice in the same second,
Python's import cache wouldn't notice that the .pyc file is stale and
would execute old code.

The 2.x docs are currently not hosted on any site :(

Add more stuff to .gitignore

README.rst: Fix link to buildout documentation