Implement session.insertAtCursor for the sandbox.

Ok! My first attempt at expanding the sandbox API ;-)

So, just some low hanging fruit here: I know an *extremely* common operation will be to insert arbitrary text wherever the cursor is, and currently that requires two API calls (one to get the cursor and one to insert the text there). This unifies that into a single API call which makes it significantly easier, with less callback hell.

This opens up the door for extensions that insert the current date & time, or any other arbitrary value, eg: http://paste.ubuntu.com/7082673/ look at how simple and elegant that is ;-)