added ability to use fontello font packages by placing new folders inside of the "fontpacks" directory.

each new folder represents a different fontello font pack set.
Move font packs registration into a new method
Update font pack class:

* Perform basic config validation before registration
* Cleanups
Prefix font pack type and label

* Type: pack-*
* Label: Pack: *
Use CSS file mod time for font pack version

To set a custom version, a "version" entry can be added to config.json
Prevent duplicate font pack registration
Cleanup doc blocks
Fix fontpack class properties
Merge pull request #21 from kucrut/fontpacks

Fontpacks