Removes categories creation from the admin section.

Issue #332.
Removes the categories from article creation, article search and article lists.

Issue #332.
Removing category support.

Note for self: next time I want to drop a complete model, the easiest, fastest way is deleting the model files and fix the spec until they work.

Issue #332
Adds category removal migration.

This migration does the following:

* Picks up each category
* Checks if a tag having the same permalink exists
* If not, the tag is created
* Every article using that category is added to the mentioned tag.
* Category model is dropped

There's no way back.

Issue #332
Merge pull request #334 from fdv/feature/drop-categories-for-tags/332

Drops categories for tags.