Module categorization

[ahutchings]

Create categories for modules. Categories might include:

Authentication
APIs
Development Tools
Events
Email
Forms
Media
Object Modeling
Database
Views & Templating

[ahutchings]

Given that Kohana is moving towards composer support, this should probably be implemented using composer.json keywords instead of manually-defined categories.

http://getcomposer.org/doc/04-schema.md#keywords

[happyDemon]

I'm including this in the next pull request.
I'm also doing keyword guessing for modules that don't come with a composer.json (I'll be scanning the repo's description for existing keywords that were indexed from composer repos)