Added averaging sensors to the couchbase cluster

The value received from the couchbase node is decimal:
```
2014-06-17 23:09:06,687 WARN  b.e.feed.AttributePollHandler [brooklyn-execmanager-ni8MYkJ6-1758]: unable to compute CouchbaseNodeImpl{id=q6N6g0fR}->Sensor: couchbase.stats.cmd.get (java.lang.Long); on val=
HttpToolResponse{responseCode=200}
java.lang.NumberFormatException: For input string: "533.5335"
        at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) ~[na:1.7.0_60]
        at java.lang.Integer.parseInt(Integer.java:492) ~[na:1.7.0_60]
```