Various bug and doc fixes

README          Cluster placement groups are supported
EC2.pm          Doc fixes, associate_route_table() did not pass RouteTableId to API call
Snapshot.pm     When copying snapshots to another region, describe_snapshots() would
                 occasionally not return a value in refresh() when in a wait_for_snapshots()
                 loop.  Now refresh() checks that a new value was returned before assigning to self and
                 returns true if a refresh occurred.
RouteTable.pm   Add route management convenience methods to match docs in VPC.pm
Subnet.pm       create_route_table() now returns RouteTable object as per docs in VPC.pm