Add a code editor for composing python filter functions in the browser.
Add facility for evaluating a function for each student's answer and sending back a filtered list of students whose function evaluates to True.

Uses multiprocessing to enforce timeouts and process isolation. As a
side effect, few functions are available inside the filter function.
Allow print statements to return output.
Merge branch 'master' of github.com:yoavfreund/Webwork_AdaptiveHints into filter_func
Merge branch 'master' of github.com:yoavfreund/Webwork_AdaptiveHints into filter_func

Conflicts:
	src/adaptive-hint-servers/rest_server/rest_server.py
Remove debug=True
Allow collapsing filter function and grouping sections on problem part page.
Bump up filter function timeout.
Return errors to user in filter functions.
Merge pull request #22 from yoavfreund/filter_func