TRUNK-1589: get folder for module web files programatically

I've added a getModuleWebFolder(...) convenience method in the WebModuleUtils class.
https://tickets.openmrs.org/browse/TRUNK-1589

Please review the code.

My questions are: 
How can I get a ServletContext from the module?

Or I can use dispatcherServlet.getServletContext() in the WebModuleUtil?
If yes, when does this dispatcherServlet equal to null?