Add basic auto-render extension

Summary:
Add an auto-render extension to render math on a page. It
exposes a global function (maybe we should attach it to `katex`?) to
render math in an element. It comes with a README on how to use it.
Also, make `make build` build the minified file.

Fixes #26

Test Plan:
 - Visit http://localhost:7936/contrib/auto-render/
 - See that all of the math renders correctly
 - `make test`

Reviewers: alpert, kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D16620

LICENSE.txt

@@ -1,6 +1,12 @@
 The MIT License (MIT)
 
-Copyright (c) 2014 Khan Academy
+Copyright (c) 2015 Khan Academy
+
+This software also uses portions of the underscore.js project, which is
+MIT licensed with the following copyright:
+
+Copyright (c) 2009-2015 Jeremy Ashkenas, DocumentCloud and Investigative
+Reporters & Editors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
@@ -18,4 +24,4 @@ FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
+SOFTWARE.