Add multi-threaded compilation support

We have a project with a large number of JSPs and JspC take forever to compile them. I basically just created a subclass of JspC that submits the processFile method call to a thread pool executor. This significantly improved our compilation time. I couldn't subclass the tomcat5 version of JspC and have access to all of the necessary variables so rather than duplicate the entire JspC class I just didn't update the compiler to support multi-threading.