CFD-202 : Split og vocab facet

Spliced the facet in 2 new facet blocks:
- Group Categories
- Group Tags

Added extra facetapi sorter so we can sort terms by their weight.

Added extra facetapi widget so we can group taxonomy terms by their level 0 terms (used for Group categories).

Added missing geo tags to discussions.

![c4m-discussions-overview](https://cloud.githubusercontent.com/assets/133124/5179890/1a53c16e-7485-11e4-9624-8fb57cb4ecba.png)



That is the part where I don't like the stdClass used everywhere in Drupal:

An id is an integer, so should the value when you retrieve it from the object. We get by default an string. That is why I always cat them to integers.

As you know in PHP:
1 == 'test' // TRUE
'1' === 1 // FALSE