URLrewriter: make it possible to set use-context to true

Some customers don't follow our recommendation and leave the context-path in the URL, it would be nice if we could configure whether the context path should be taken into account. Doing this on a site-wide basis should be fine.

The attribute use-context="true"[0] (the default is false) can be set on the urlrewrite element.

The default should ofcourse remain like it is now.

[0] http://urlrewritefilter.googlecode.com/svn/trunk/src/doc/manual/3.2/index.html