Various bug and documentation 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
Fixes for latency in objects registering and when request limit is exceeded

New objects don't always appear immediately after request.  Use eval loop
to wait for them to appear.
When RequestLimitExceeded error is returned by API call, go into exponential
backoff retry loop for up to just over a minute.
Merge pull request #12 from lkinley/master

Various bug and doc fixes