Mounts table for OSX

Associated with #255, this adds Mounts table support for Darwin.

The information is gathered using `getmntinfo()`. Currently it has somewhat different info compared to what the Mounts table for linux should be showing. This is partially due to the fact that the linux table uses a different method for gathering the data, the linux table segfaults for me, and I haven't reconciled the content in `statfs` between linux and OSX to see what the differences are in available data.
Can you change these to be all-lowercase snake case?