Allows use of label tag

Activates minimalect when a label with for="select-id" attribute is
clicked, as in native form behavior.
Add tabindex from original select element

Adds the tabindex from the native select element, if present, to the
minimalect input. Improves tabbing through the form.
Prevent enter key from submitting the form

Forms got submitted when selecting an option with the Enter key because
the "keyup" event fired after the form submit event. Changed listener
to "keydown".
Close minimalect when tabbing out of it

It would however be more useful to find why it doesn't close in the
first place (except when tabbing into another minimalect).
Merge pull request #23 from Frique/master

A bunch of usability fixes