Made "mobile" a boolean option when instantiating the selectpicker

Currently you have to make two calls to create a mobile-friendly select: 

1. Instantiate bootstrap-select, e.g. `$('.selectpicker').selectpicker();`
1. Mobilify the select, e.g. `$('.selectpicker').mobile();`

I made a small change so `mobile` is an option that can be passed in when instantiating (defaults to `false`, of course), e.g. `$('.selectpicker').selectpicker({mobile : true});`

Here's a [fiddle](http://jsfiddle.net/jkrehm/4UjVr/) to illustrate the change (view it on a phone/tablet).

I did not update the documents branch with the new option.