Separated `setOptions` logic into two parts:

 - updating underlying SELECT element options (stayed in setOptions)
 - updating SelectBox control based on the SELECT options (moved to
`refresh` method)
 
Method `refresh` now contains all the logic necessary to refresh the
control and no longer uses `setOptions`. Method `setOptions` is calling
`refresh` to update the SelectBox control.

Fixes marcj/jquery-selectBox#88.