adds fstab plugin

This adds a plugin to expose fstab entries in the same way the filesystem plugin exposes mounted filesystems. Example output here https://gist.github.com/rillip3/72fbcb86b51b46ad202f
I think it would be better to read the file using built-in methods rather than cat'ing a file to stdout and parsing stdout. See https://github.com/rackerlabs/ohai-plugins/blob/master/plugins/login.rb#L10-L18 for an example.