(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 was unable to test if this test case actually functions, as OS X does not seem to support the required Native GDAL libraries for OGR. Additionally, I think this particular test file is not configured to run under mvn test (?).