Added 'selleckt' to the default element classnames

This makes it easier to differentiate selleckts on the page - the current value, 'dropdown', is a bit too likely to cause clashes IMO (I've kept it though for backward-compatibility)
Updated demos to use .selleckt classname instead of .dropdown
Merge pull request #53 from spmason/default-classname-selleckt

Added 'selleckt' to the default element classnames