allow passing arguments to tox

e.g. "tox -- scrapy.tests.test_contrib_loader" runs test_contrib_loader for Python 2.6 and 2.7
use tox deps instead of "manual" pip install; add lucid and precise environments to tox.ini
add django to test requirements and fix test_djangoitem for modern django versions
make tox test running more consistent with bin/runtests.sh script
reuse runtests.sh and runtests.bat scripts in tox.ini
allow passing arguments to runtests script via tox
Merge pull request #341 from kmike/test-running

Improved tox test running & fixed django tests