Adds a string interning pass to the compile process.
One variable per unique string literal is added to a JsVars at the top of the program.

Patch by: bobv
Review by: scottb
Suggested by: bruce



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1475 8db76d5a-ed1c-0410-87a9-c151d255dfc7