document optional bootstrap/angular-bootstrap dependencies

[joemfb]

These dependencies are mentioned (https://joemfb.github.io/ml-search-ng/ml-input.html) but should be more prominently covered in the README

re #100, #99

[joemfb]

Note, the angular-bootstrap typeahead features in ml-input are now prefixed with uib, and appear to require v0.14.0 or newer: https://github.com/angular-ui/bootstrap/blob/master/CHANGELOG.md#0140-2015-10-09.