css tweaks
made output model an optional parameter
added maxHeight  and defaultLabel
    buttonLabel now refers to a property  of the currently selected item to be used on the button (only for single selects)
Changed the filter model to be on an object
Added multi-select class to main element
Bugfixes
Merge remote-tracking branch 'origin/master'
added defaultLabel to demo
Added buttonCallback (#2)
     This will allow total customization of the button whenever an item is selected.
     When this functino is defined, it will be used instead of buttonLabel
Introduced search-properties attribute (#3):
     This accepts an array of property names to be searched by
     If this attribute is omitted/is empty, the default filter by all fields will be used
output-model will no longer list invisible items
removed an unnecessary  error occuring  whenever no output-model was defined
Merge pull request #5 from s-hoff/master

merge with master