Wrap bootbox as an UMD module

Following #180, we now have an UMD module for Bootbox
### TODO ###
 * adapt jshint configuration so that it doesn't complain about `require`, `define`, and `module`
 * find out what the `bootbox.init()` function is for and, if it is only used for testing, try to find a way to make tests work without it
 * write tests that use the AMD API