Search suggestions

The algorithm works roughly as follows:
 - do some string matching
 - weight according to how many articles the stream for that tag will contain
 - pick the first section, region, person, organisation and topic matching, in that order
 - show more topics, regions etc until reach 150 characters 

![image](https://cloud.githubusercontent.com/assets/447559/5142167/3014c9dc-7177-11e4-99e8-7a68ce037dbb.png)