add directory space use monitoring

[tcrayford]

this can be useful if/when you want to ensure a specific directory
doesn't go over a certain size, or if you want to monitor a particular
directory's size.

I use it in production to monitor a few key directories, (with no
alerts), and when disk space is critical I use coalesce (with these
metrics and the riemann health disk space metric) to get an alert
description that includes the space use of said key directories (so I
can know it's e.g. graphite filling up, or /var/log filling up, or riak
filling up the disk

[aphyr]

Awesome :D