This request adds hints to the sensor UI.

For now, this uses a static list of sensors, which we can update occasionally.   
                                                                                 
This is mostly because OpenTSDB only supports pulling 25 sensors at a time from  
its API (the /suggest URL). To be able to get all of them will require patching  
OpenTSDB. 

Also note the patch to Bootstrap's typeahead support. This allows showing the
hints when no characters have been typed yet.