accounted for 3-digit case in use_pct and spaces in file name

Reviewers: jace

Differential Revision: http://phabricator.khanacademy.org/D2479
Just copying over learning_analysis.py from khan/webapp with an extra note in the initial docstring about the ipython notebook. Also added ipython notebook.

Test Plan: Reran script and passed results to ipython notebook, new results match the old results.
Finalized script for analysis of experiments like TopicModeAccuracy and EarlyProficiency

Test Plan: Ran TopicModeAccuracy data retrieved with an interactive hive cluster through this and plotted data in an ipython notebook, verified that learning/ability gain curves were reasonable as well as the relationship between time spent on each of the alternatives in the experiment