Adds a gwt-compiler subproject and build rules. This subproject ensures that a compiler core is buildable without including hosted mode infrastructure. This could be useful in the future for things like embedding the GWT compiler into a server or other tools. Review by: bobv git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5875 8db76d5a-ed1c-0410-87a9-c151d255dfc7