Add a mvel URL handler to fabric-core + quick doc

For one of my use case, I need to be able to setup a specific jetty SSLConnector for my https connection. To do so, I need to create a specific jetty xml file with my connector and the port to which I want to use. THe problem is that pax-web also use a specific property to specify the port number.
One of the tweak that was done in fabric8 was to use a 0 port value so that pax-web overwrite the conf with the good port value. This works for standard http connection but as far as the https connection is concerned this does not work.

After discussion with @iocanel it was decided to create a generic mvel URL handler to do it... and more if needed :-) .

Best,
Jerome