feat(select): added config option to show the clearbutton.
The clear button can now be always displayed with the option config.showAlwaysClear
Its default is false.

close #16
Added option to hide checkboxes during single selection. Default to false. Added button in test.html to test feature.
rename css class
add styling to less file
rename config property, add comment and correct button label in test.html
feat(clear button): add config property 'showClearAlways' to show clear button always, even if field is required

Closes #16
feat(checkboxes): add config property 'hideCheckboxes' to hide checkboxes in single selection mode

Closes #13