[GEOS-6672] Creation of a WPS Process for mbtiles generation

Please look at the following JIRA: https://jira.codehaus.org/browse/GEOS-6672. I have also modified the MBtiles Output format page describing the WPS process parameters.
What about accepting a collection of layer names? Thinking that most mbtiles use will be for generating base maps consisting of multiple layers. Or is the idea to force the user to create a layer group first?