Adds nfs client collector NfsCollector based on NfsdCollector

I had a look at the nfsd collector, and created one for nfs clients. I'm using CentOS and RHEL 5/6 that I've based the metrics on what nfsstat pulls out of /proc/net/rpc/nfs. The ordering in RHEL's /proc/net/rpc/nfs file is quite different from the nfsd collector already included in Diamond -- not sure what OS it was written for.

I've added a bunch of try blocks for nfs v4 stats greater than 30. I've yet to run across a server that doesn't at least have the first 30. 

The test script nfstest.py definitely needs some review. I've only tweaked what I found in the nfsd test script, so I am not entirely sure if the metric values will still be accurate for nfs clients.