Add support for single empty message when working with multiple datasets...

[1django]

.... Addresses #780 (hopefully).

As per a suggestion in the issue comments, when you are using multiple datasets, but only want to display a single empty message (instead of one per dataset), construct the typeahead like so:

$('.typeahead').typeahead({
  /* other options */
  sharedTemplates: {
    empty: '<p>Some informative empty message.</p>'
  }
});

This pull request passes npm tests, builds fine, and works for my project. However, I'm guessing there may be better ways to accomplish this...Probably due to my lack of understanding around some of the render and update related events and methods.

I'd love to see how to do this properly...

Oh, and also, I did not add a new test for the global empty message, but holler if that's required to consider this PR.

[xandor-io]

This is really helpful, have you fix the error in order to merge?

[devgnx]

https://github.com/twitter/typeahead.js/issues/780

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