Clytwynec/har

@jzoldak  Please review.

The new functionality is added as a subclass of WebAppTest, so nothing is changed in web_app_test.py.  The only new changes affecting the existing functionality are in browser.py and the requirements.txt.  

Right now the tests for the new module are commented out, since the java app is needed.  (I did run them locally.)

For reference, here are the current changes in the edx-platform for how its used there:
https://github.com/edx/edx-platform/commit/9911f8c9d2d61b4c2675340dce3ffdd60adb6004 

And here are the configuration repo changes:
https://github.com/edx/configuration/commit/f55d25bdd605d38e01d6c11bc6a3c50db73f9905
I'm not sure I understand how this would be used.  What is the advantage of using a cache?