Reduces dropdown flicker during a remote search.

This provides a fix for [#176](/../../issues/176).

The behaviour of Bloodhound has been changed such that the
 #get callback is only run if matches in the search index
 (i.e. local or prefetched) have been found, or if we're
 not making a network request.

The behaviour of Typeahead has been changed such that the
dropdown is only cleared by the query changing if the query
becomes an empty string.