Label behaviour + 'refresh' improvement + minor fixes

• 'refresh' method now updates only '.dk_options_inner', so the options
may be refreshed even with the dropdown opened
• Clicking a label now focus the respective select dropkick
• Abillity to set a placeholder that is hidden from the options by
setting 'disabled selected' to the first select option
• Corrects border-radius when the dropkick opens on top by adding
'.dk_open_top' class to container []
• This version of DropKick was tested and confirmed that works with
jQuery 1.7+