Update src/directives/typeahead.js

I propose this modification to let typeahead manage functions as data 
source (see on http://twitter.github.com/bootstrap/javascript.html#typeahead 
function as source parameter).
Without the modification if you assign a function as bs-typeahead parameter
it doesn't work (an example on http://plnkr.co/edit/WbjtvcShroFHyxOpAeVb)
A working example with the modification in http://plnkr.co/edit/QY94ji9kGSeyQos5KfWC