Finished off tests for tags

This also adds back the ability for tags to return `null` in order
for no tag to be added to the results.
Added tests for the single selection container
Fix hiding search

When the dropdown is in the dropdown that is attached to the body,
the call to `show` causes `display: inline` to be set directly on
the search container. This causes rendering issues, so this change
uses a class instead of calling `show` and `hide` directly.

This is most likely a jQuery bug of some form, as it only happens
when using the dropdown that is attached to the body.
Add tests for the multiple selection container
Added `maximumInputLength` support