Add options to set base button- and drop-wrap-class.

Hi,

I was working on a project which is currently not using bootstrap and some classes seems to collide. So I've added two more options to set the base class of the generated button and the class.

As an example:

´´´
$('select').selectpicker({
    buttonBase: 'bs-select-control',
    dropBase: 'bs-select-group',
    style: 'bs-style-default'
});
´´´

Sorry for bumping the version number and compiling all the stuff but I needed it in case you won't merge it in your repo so I can rely on my fork for bower.

Just let me know if I can improve something to get this merged into your codebase. Thanks. :blush: