Alter typeahead to accept synchronous/asynchronous data source

Created by: mlmorg

@fat: From existing pull request (#3250) against 2.1.0-wip. Sorry for not submitting it against a *-wip branch earlier.

This is a simple change to allow both synchronous and asynchronous data sources for the typeahead widget via a function/callback. This references issue #1336 (closed). Example:

$("input").typeahead({
  source: function (query, process) {
    $.get('/autocomplete', { q: query }, function (data) {
      process(data)
    })
  }
})

Because of the recently-added "updater" option, you can also handle objects:

var labels
  , mapped
$("input").typeahead({
  source: function (query, process) {
    $.get('/autocomplete', { q: query }, function (data) {
      labels = []
      mapped = {}

      $.each(data, function (i, item) {
        mapped[item.label] = item.value
        labels.push(item.label)
      })

      process(labels)
    })
  }
, updater: function (item) {
    return mapped[item]
  }
})