(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. 
Might want to add a test that users both limit and offset, just to cover also the combination case.