Allow minSearch to be 0 and open on down arrow

[cindoum]

Fix minSearch to accept 0 as value and fix the underlying fonctions.

[pkozlowski-opensource]

@cindoum I understand what you are trying to do but you are choosing a particular, arbitrary approach and we've got users coming with different use-cases (for example - trigger on focus and not on the key+arrow down). The proper solution is to refactor this directive in order to extract a directive's controller and allow people to add custom logic by extending the base typeahead directive. An attempt is here: #888

I'm afraid that this PR can't be merged as-is.