added FeatureCount for JSON-OutputFormat

Hi GeoServer-Developers,

I have just extended the GeoJSONOutput-format with an extra 'totalFeatures' attribute, that shows the featureCount for a request.

I made this patch in agreement with the geoserver developer list (see thread http://osgeo-org.1560.x6.nabble.com/JSON-output-total-number-of-features-for-paging-td5035093.html)

I have also created the test "testGetFeatureCount() in "GeoJSONTest.java" which shows the featureCount in a request without a filter and a request with a filter.

I would be glad if you could implement the patch into the main branch.

Best regards
Paul