Add feature to filter issues by multiple labels

The idea of this feature is to have the possibility of filtering issues by more than one label. Only issues which contain all the selected labels should be displayed.

Feature Request: http://feedback.gitlab.com/forums/176466-general/suggestions/4036826-multiple-label-selection

#### User Interface
The implementation consists of a modified UI for a project's issues page:
* labels dropdown filter removed
* all project's labels displayed in left sidebar
* selected labels are highlighted as shown

Screenshot before:
![labels_filter_before_screenshot](https://f.cloud.github.com/assets/2506871/2195990/a937030a-98a8-11e3-8a8e-6aa437606067.png)

Screenshot after:
![labels_filter_screenshot](https://f.cloud.github.com/assets/2506871/2195800/a1d0ca72-98a5-11e3-8f6b-087d453ae29e.png)


#### Implementation
* selected labels are added to ```params[:label_name]``` and separated by commas
* because of the way *acts-as-taggable-on*'s ```#tagged_with``` method works by default, no other backend modification is required
* normal links add the label to the filter
* highlighted links remove the label from the filter

#### Testing
A Spinach feature was written to test the functionality.