Issue 517 categories

This pull-request introduces a big change into PyBossa. Basically, the pull-request add the feature Categories for applications.

Any application can have one and only one Category. By default, the system provides the migration files (see Alembic versions) creating a default Category: Thinking (the usual type of applications for this type of servers) and assigns it to all applications that have at least 1 task and a task presenter. 

The categories can be modified/managed only by admins of the system. Thus, they can change the name of the categories, add to or delete from the system.
![captura de pantalla de 2013-05-24 13 37 57](https://f.cloud.github.com/assets/131838/560074/6d3820d0-c466-11e2-93af-f088a6fdde11.png)


The previous categories: draft and published are gone in the way they were managed in the past. Basically, all published apps are now assigned to the default category: thinking. In case you want to revert the name to Published, as an admin, click in Admin site->Manage categories and change the name to published. All the apps will automatically reflect this change.
![captura de pantalla de 2013-05-24 13 37 18](https://f.cloud.github.com/assets/131838/560071/5382a2c8-c466-11e2-870b-952d050a10d9.png)


Draft apps have been now hidden for everyone except for the owners and the admins of the site, so they can contact the developers and help them to finish the applications.