Ensure element is select before running function

I was running into issues with a blank select being created when the `render` and `refresh` methods were run without specifying the `.selectpicker` class. This was happening because IDs and classes are automatically transferred to the new selectpicker. This just checks to make sure the element is a select - if not, the selectpicker function doesn't run.

I also fixed min-height, so it adjusts to the number of options if less than 3.