(GEOT-4963) Added fix for ContentFeatureStore.getCount() with startIndex...

Fix for ContentFeatureStore.getCount() with startIndex or MaxFeatures set (https://jira.codehaus.org/browse/GEOT-4963).
Modified test cases in ContentFeatureStore to test for getCount()
Added test cases to applicible extending classes. 
I think you forgot to check if "the internal actually counted", like the comment on the previous block says, this might result in returning 0 where you should return -1