Add JSONP-style onload "handler" to JsServlet

Particularly for container JavaScript but in other cases as well, clients loading JavaScript from Shindig occasionally do so using dynamic loading. When doing so, they need to know when the servlet's JS has completed loading, which is difficult to impossible to do across all browsers consistently in this case.

This CL adds support for the "onload" query parameter to JsServlet. Its value is the name of a method defined globally that indicates loading has completed, the call appended to the tail of the request. The param is sanitized by a simple regular expression and JS escaping.

[Paul Lindner, 2010-05-26 18:18:33]

lgtm