Allow to use different ExpressionFactory implementation

It should be possible to use a different ExpressionFactory implementation. This feature is required
to use 3rd party EL implementations like JBoss EL. Mojarra already supports this with the
'com.sun.faces.expressionFactory' context parameter.

MyFaces Core 1.2.x already supports a context parameter 'org.apache.myfaces.EXPRESSION_FACTORY'
but it is only evaluated in a JSP 2.0 environment (see MYFACES-1693 for details).

It should be possible to use this parameter with JSP 2.1 as well. The corresponding code
should be refactored from Jsp20FacesInitializer to AbstractFacesInitializer to be usable in both
JSP 2.0 and 2.1.

Discussion on myfaces-users:

http://www.nabble.com/Replacing-expression-factory-td18867420.html