Import portions of the Closure Library.

Source was checked out from https://code.google.com/p/closure-library/,
at revision 6a51959b7f732ee1f568eaf8dfde0522b8d2d0c2 (2014-04-23).  Only
copy the library modules that will be needed for the extension behavior
code in the Apps & Extensions Developer Tool.
Load Closure Library code in main.html.
Load the watchdog.Activity module in main.html.

watchdog.Activity provides helper code to work with extension and app
behavior logs.
Merge remote-tracking branch 'origin/master' into closure

Conflicts (manually resolved):
	main.html
Merge pull request #56 from mvrable/closure

Closure