Add getGrammarTypeForPath mobile API

Summary:
Add an API endpoint for getting the grammar type that a widget should use on the
mobile app. Add documentation for how it works. Also, improve the documentation
for `getDOMNodeForPath` to match what we currently do (there's a fallback if it
isn't implemented).

Test Plan:
 - Enable staticRender on the testing perseus page
 - Make an item with all of the widgets that implement the mobile API
 - Turn on the tick controller on number-line, enable 'formal with movement' on
   the transformer and add one of each of the transformations.
 - Run
```
itemRenderer.getInputPaths().map(function(x) { return itemRenderer.getGrammarTypeForPath(x); })
```
   - See that it doesn't break
   - See that every item in the list that it returns is either `"expression"` or
     `"number"` (in particular, everything should be `"number"` except for the
     expression widget).

Reviewers: aria, charlie

Reviewed By: aria, charlie

Subscribers: charlie

Differential Revision: http://phabricator.khanacademy.org/D13846