Show dropdown with default suggestions if minLength is 0

When minLength is 0, dropdown will be shown on focus and when opened
with $(...).typeahead('open')

Also adds an all method to SearchIndex
Merge pull request #719 from azavea/default-suggestions

Show dropdown with default suggestions if minLength is 0

Conflicts:
	src/bloodhound/bloodhound.js
	src/typeahead/typeahead.js