Merge pull request #50 from NotBobTheBuilder/master

Update typeahead when model set to null/undefined
Added optional suggestionKey attribute to directive

The suggestionKey allows the model that the directive is bound to
to be updated to a value contained on the suggestion model, without
the new model being the entire suggestion model itself.
Updated README with options section, to include suggestionKey
Merge pull request #52 from mikesir87/master

Ability to change value set on model, based on suggestion