Add cat enum field to RoleSkill

[joshsmith]

We're missing the cat enum field for RoleSkill.

For #26.

[npendery]

@joshsmith what exactly does this do? I found something that I think we could use for it https://github.com/gjaldon/ecto_enum

[joshsmith]

@npendery we've just been using validate_inclusion and treating the column as a string for now. Maybe later we can circle back and add database-level integrity by using PG's enum or something like this.