New features & bug fixes

I added the `hideDisabled` option (`data-hide-disabled` attribute can be used as well). It is set to `false` by default. When set to `true`, disabled options and optgroups are not displayed in the menu. This is a feature included in Chosen (#129) that I thought would be good to incorporate.

I also added the ability to select an option using the enter key, so the keyboard can be used entirely. Before, an option could be focused using the keyboard, but would still have to be selected using the pointer. (#69)

I fixed an issue that has been bugging me for a while. When a select had an ID, it was transferred to the button as well so clicking on the label would focus on the button. However, this means there were always two elements on the page with the same ID, which isn't ideal for a variety of reasons. Now, the ID is added to the button via the `data-id` attribute. This can be accessed via CSS using 

````CSS 
[data-id="selectID"]