`setOptions` now depends on `refresh` and not vice-versa

Separated `setOptions` method 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.