add tox configuration file for easy testing

.gitignore

@@ -25,3 +25,4 @@ updates_key.pem
 *.mp4
 *.part
 test/testdata
+.tox

tox.ini

@@ -0,0 +1,5 @@
+[tox]
+envlist = py26,py27,py33
+[testenv]
+deps = nose
+commands = nosetests --with-coverage --cover-package=youtube_dl --cover-html --verbose test