Typeahead: Prevent lookup until user finishes typing

A fix for bug #573, though not through modifying typeahead-wait-ms. Instead, I've added typeahead-throttle-ms, so that:

    <input typeahead="item in getAsync($viewValue) | filter:$viewValue" typahead-throttle-ms="500">

will only send one request if the user types a string in under 500 milliseconds. this should work in conjunction with wait-ms, for what it's worth. 

Thoughts/suggestions?