Refactor the plugin core and fixed #1.

- There are two classes now: `SingleSelectLisbox` and `MultiSelectListbox`,
  which are inherited from the `Listbox` class.
- Fixed #1: use selection of the parent element during initialization.
Removed `multiselect` option.

`multiselect` option doesn't used now.  Use `multiple` attribute of
the `<select>` tag instead.
Fix some bugs and make some comments.
Adopt README to the current status.