Added tox.ini

tox.ini

+[tox]
+downloadcache = {toxworkdir}/cache/
+envlist = python2.6-django1.5,python2.6-django1.4,python2.6-django1.3
+
+[testenv]
+commands = {envpython} rest_framework/runtests/runtests.py
+
+[testenv:python2.6-django1.5]
+basepython = python2.6
+deps = https://github.com/django/django/zipball/master
+
+[testenv:python2.6-django1.4]
+basepython = python2.6
+deps = django>=1.4,<1.5
+
+[testenv:python2.6-django1.3]
+basepython = python2.6
+deps = django>=1.3,<1.4