add CG channel and group management methods
add CG namespace management methods
add ChannelGroups PAM & tests
add channelGroupHereNow() method
add setChannelGroupState() method
add ###version### template in Pubnub.php
refactor urlencode usage
remove Client::encode*rs
fix version templates
add channelGroupSubscribe() method
fix subscribe() method
remove support of PHP <= 5.2.x
remove docs for 5.2
bump version to 3.7.0
merge add-cg feature branch