Added pytest-cache to tox.ini

This allows you to only run the last failed tests using `py.test --lf`
or `tox -- --lf`.

.gitignore

@@ -11,3 +11,4 @@
 /dist/
 jedi.egg-info/
 record.json
+/.cache/

tox.ini

@@ -3,6 +3,7 @@ envlist = py26, py27, py32, py33, py34
 [testenv]
 deps =
     pytest>=2.3.5
+    pytest-cache
 # docopt for sith doctests
     docopt
 # coloroma for colored debug output