Add test for getHosts() method and fix a few unit test issues:

do not re-declare $db in MongoGridFS because it already extends MongoCollection. test was broken

fix MongoClient tests to not rely on the unsupported public $server variable

fix MongoCollection::group test to match the actual spec
only use persistent pfsockopen for HHVM, since PHP still has issues with it for some reason
fix hhvm array access error
do not request replica set config or status unless replSet is specified, since it would be a waste of a request and slow down performance for non-replica set configurations.
Update README.md

add note about replica sets using APC