virtualenv creation files sometimes for non-default python environment

Issue #18 on hold
created an issue

VERSIONS: tox 1.0, tox 1.2, virtualenv 1.6.4

OS: MACOSX 10.6, 10.7

PRECONDITION: py2x python interpreter is not the default python interpreter.

PRECONDITION: virtualenv for py2X is not created yet

The creation of the virtualenv environment fails sometimes. Currently, tox uses the following syntax (which uses from the default-python site-packages directory):

{{{ /absolute-path/to/python2.X /absolute-path/to/python/lib/site-packages/ ... py2X ... }}}

SHOULD-BE: {{{ /absolute-path/to/python2.X /absolute-path/to/python2.X/lib/site-packages/ ... py2X ... }}}

FAILING EXAMPLE: Python 2.6 is the default python interpreter, py27 should be created on MACOSX. A readline package was installed where was located. This lead (for me) to an assertion error in the readline package, which caused the virtualenv creation to fail.

NOTE: After I removed, everything worked again. But I assume there exist other cases. In addition, I think the SHOULD-BE case is slightly more correct. But it assumes that virtualenv is installed for all Python versions under test.