Added search wiki feature

As a lot of people requested, I implemented the search in wiki pages.(again), hope this time I followed the style guid and contributing guidlines... 
Refers to:
http://feedback.gitlab.com/forums/176466-general/suggestions/4078088-add-wiki-search-function

Comments:
Since the structure of gitlab has massively improved since the last time, it was now much easier to implement the search feature. I simply added a new filter to project_filter called Wiki which calls the search engine with a new scope 'wiki_blobs'. This is evaluated in ProjectSearchResults, where a new method works straight on the ProjectWiki repository to search files.

New UI:
![image](https://cloud.githubusercontent.com/assets/4696860/4163659/0ee2e6dc-34ea-11e4-9b34-b143e1333184.png)


Affected files:
app/controllers/search_controller.rb
app/views/search/_project_filter.html.haml
 lib/gitlab/project_search_results.rb
New file:
app/views/search/results/_wiki_blob.html.haml