TNL-731: Add possibility to search notes.

**Ticket:** [TNL-731](https://openedx.atlassian.net/browse/TNL-731)
**sandbox:** http://tymofij.m.sandbox.edx.org/courses/edX/DemoX/Demo_Course/edxnotes/

@explorerleslie,
@tymofij , @olmar , @jmclaus please review.
@polesye I think we should not use it without method names, as it will throw an error with most recent version and that we will eventually upgrade. Check annotated source code:

http://underscorejs.org/docs/underscore.html