organisation: added model for working groups

Recent development has revealed the need for working groups being
managed by an app or a plugin. This will allow local groups to be
linked to working groups. Working groups are attachable to themes
in the same way as projects are.
organisation: add basic view for working groups

Pretty straightforward except for maybe the boostrap templatetag
library which adds a way to split an object list into rows to make
it easier to use bootstrap's grid layout.
organisation: style the working group templates

This uses the new feature blocks to show the workign groups along
with their logos. Removed the bootstrap template tag as we now only
show one per line.

The current icons used for the working groups have a specific
background which we hardcode into the style sheet. It would
have been better to have an image without a background but this
goes in as is to speed up the working group insertion.
organisation: add tests for working groups
search: add working groups to search
search: Consolidate search results and fix urls

Urls for press must be cast into a context variable since we do not
know if press pages have been added to the cms.
pep8: Fix pep8 compatibility
Merge pull request #107 from okfn/working-groups

Working groups