Code Snippet management

For Issue #162

This adds support for Code Snippets via Alcatraz.  It follows the format of Templates, so that you are installing a collection of Snippets rather than single Snippet.

To solve the issue where UUIDs can collide when installing Snippets, I just generate a new one and install.  The changes of collisions is very low. We could make it guaranteed to not collide by checking against all the snippets already installed.

Small Note: I think there is room for improvement to make adding another Package type to the UI simpler, but I was impressed on how simple it was to add the Snippets.
@maschall better to replace them all that don't match the license. [This one matches.](https://github.com/supermarin/Alcatraz/blob/master/Alcatraz/Controllers/ATZPluginWindowController.m)