Update src/directives/typeahead.js

I propose this modification to let typeahead manage functions as data 
source, so the data set could be dynamically generated for example executing a query to a restful service (see on http://twitter.github.com/bootstrap/javascript.html#typeahead 
function as source parameter).
Without the modification if you assign a function as a value for the bs-typeahead attribute
the drop-down list doesn't show any value (an example at http://plnkr.co/edit/WbjtvcShroFHyxOpAeVb)
A working example with the modification can be found at http://plnkr.co/edit/QY94ji9kGSeyQos5KfWC