Make JSP 2.1 optional

Myfaces 1.2.0 requires JSP 2.1 to be present. Otherwise the StartupServletContextListener fails with this error:

Exception sending context initialized event to listener instance of class org.apache.myfaces.webapp.StartupServletContextListener
java.lang.NoSuchMethodError: javax.servlet.jsp.JspFactory.getJspApplicationContext(Ljavax/servlet/ServletContext;)Ljavax/servlet/jsp/JspApplicationContext;
at org.apache.myfaces.webapp.DefaultFacesInitializer.initFaces(DefaultFacesInitializer.java:102)
at org.apache.myfaces.webapp.StartupServletContextListener.contextInitialized(StartupServletContextListener.java:57)

JSP version 2.1 or better JSP in general should be an optional dependency, like discussed here and implemented in Sun RI too:
http://www.nabble.com/Does-MyFaces-1.2-require-JSP-2.1--tf4112432.html#a11693501

Use cases:

• Run Myfaces 1.2.0 in JEE 1.4 environment (Tomcat 5.x).
• Easier setup and smaller distributions when running Myfaces in an embedded servlet container (jetty) with facelets. For example I'm using jetty for junit tests. I haven't found a working setup which solves the jsp problems.