Added support for global GRUB configuration

This patch adds support for grub_menuentry which provides the ability to
manage individual Menu entries for both GRUB Legacy and GRUB2.

Testing has been focused on Linux-based operating systems and it is
possible that features may be missing for chainloaded operating systems.

Support for managing GRUB2 users has also been added. This compares
against the *running* configuration, not what is saved in the
compilation files in grub.d.

The following custom types were created:
  * grub_config => Manages global GRUB settings
  * grub_menuentry => Manages GRUB menuentries
  * grub_user => Manages GRUB2 users

Fixes voxpupuli#10