#65 Master 65 virtualtable encoder support

I give here a quick summary:
- add a new GSVirtualTableEncoder + related classes & enums
- set deprecated the GSFeatureTypeEncoder methods add/setMetadata and link to the new methods add/setMetadataDimension
- add GSFeatureTypeEncoder methods for virtual table
- GSResourceEncoder: additional methods to set/get the nativeName.
Note: When setting a sql view, the name of the virtual table should be the same as the featureType nativeName. By default, GS uses the Name to set the nativeName. By specifying the nativeName, a user can set different names for the sqlview and the featuretype name.
- add tests & example in javadoc

For the tests & example, i've used the following Geoserver example: http://docs.geoserver.org/latest/en/user/data/database/sqlview.html#parameterizing-sql-views