Initial implementation of a mountstats collector

This is an initial implementation of a mountstats collector. It's my first stab at a Diamond collector, so it's a little rough around the edges.

The function of this plugin is to parse the detailed per-mount NFS performance statistics provided by `/proc/self/mountstats` (reads, writes, remote procedure call count/latency, etc.) and provide counters to Diamond. Filesystems may be excluded using a regular expression filter, like the existing disk check collectors. Basic unit tests are included.