filesystem: creating the directories and files

filesystem: creating the directories and files

This commit adds in the filesystem subpackage. The goal of this package
is to provide and interface for adding to and removing from the metadata
storage for a given filesystem. This is primarily done in filesystem.go.
To facilitate this functionality, mountpoint.go exposes an interface for
querying the system about the current mounted filesystems and their
information. Note that this operation is done with a lazy loading
mechanism.

To refer to other filesystems, we use link files that can be parsed by
libblkid. The README is also updated to account for this new dependancy.

This package uses the FSError type under the hood so that error messages
will include the filesystem name, but callers can still check for
specific error instances.

Change-Id: I74fe4e84b8e3a5b73f1337c35307ffe0bf7cdea9