Move Actions to use UUID instead of Sequence
Review fixes: 1. use + instead of fmt.Sprintf, and 2. simplify use of splitId
Safe parsing of tags with no production code calls to panic'ing functions
Merge pull request #31 from johnweldon/0-action-sequence-to-uuid

Refactor ActionsTag to use simpler Prefix and Suffix.

Preparation for moving Actions to use UUID instead of sequences.
Actions :: Update ActionTag with UUID structure

Now that Actions use a UUID as the ID; update ActionTag's to better
represent the Action.
Use strongly typed UUID's

    Also use utils.IsValidUUIDString
    Updates: ActionTag, EnvironTag
EnvironTag is not ready for strongly typed UUID

    This reverses 891e2be8c81f374ea745f449a5d766493e96cd3d for
    the EnvironTag.

    See http://paste.ubuntu.com/9149684/ for the issues with
    default initialized EnvironTag's (all zeros).
Merge pull request #32 from johnweldon/0-action-tags-cleanup

Actions :: Update ActionTag with UUID structure
Introduce DiskTag

DiskTag will be used to uniquely identify disks (block devices),
either attached or unattached. DiskTag's format is disk-<seq>.
Merge pull request #33 from juju/names-disktag

Introduce DiskTag
Adding logic for charm tags.
Changed year and export case based on review.
Split line for readability.
Removed redundant helpers.
Commented exported tho self-explanatory properties and methods.
Merge pull request #34 from anastasiamac/charm-tags

Adding logic for charm tags.
Renamed variable for clarity. Exporting useful regexes.
One var block for readability.
One var block for readability.
One var block for readability.