Ensure all finalizations are run when one fails

Fixes issue287.
remove testing of xdist+genscript -- doesn't really make sense
because for installing pytest-xdist you need pytest installed
which defeats the purpose of genscript.
add py33-xdist to tox testing
removing pexpect from general dependencies because
it doesn't install on windows anymore.  Instead
to specific configurations for pexpect on py27 and py33
which only call the tests that need it.
fix issue221 - handle importing of namespace-package with no
__init__.py properly. (This is a commit after the fix -- the
original issue steps for failure cannot be reproduced anymore).
Re-raise the first exception instead of the last

This will make more sense if multiple fixtures depend on each other.
It would be better if all exceptions could be shown however.

Also depend on python 2.5+ exception hierarchy and use sys module
directly.
Merge
Fixup changelog

This seems to have gone wrong in the merge.
some amendments/fixes to changelog
move two fixture test modules into bigger testing/python/fixture.py
addresses issue246: add a test for module/function scope that shows that
finalizer ordering is wrong.
refactor internal FixtureRequest handling to avoid monkeypatching.
One of the positive user-facing effects is that the "request" object
can now be used in closures.
fix py25 compat
avoid maintaining a fixturestack
remove _fixturestack attribute now that we have a proper request->subrequest->subrequest chain.
fixed version comparison in pytest.importskip(modname, minverstring)
fix ordering of finalizers of parametrized interdependent fixtures.
This fixes issue246 as reported.  Thanks Ralph Schmitt for the
precise failure example.
remove an old duplicate marker and use recent pytest mechanism for parametrization
simplify basedir isolation
fix issue357 - special case "-k" expressions to allow for
filtering with simple strings that are not valid python expressions.
Examples: "-k 1.3" matches all tests parametrized with 1.3.
"-k None" filters all tests that have "None" in their name
and conversely "-k 'not None'".
Previously these examples would raise syntax errors.

Also add a note to the docs about what is allowed.