Keyboard usability/acessibility patches

Hi, Marghoob!

I've made some changes in the plugin so the user can focus, blur, filter and open the dropdown just by using the keyboard. Also, I've fixed some bugs and implemented focus/blur event handling. 

I've tried many ways of firing these "new" events consistently across the browsers we support (Chrome, FF, IE) whithout breaking keyboard usability, but the best I could get so far still has a bug in IE: the events are triggered unnecessarily if the filter box is used. At least they are always triggered in the right order, so I decided to leave it that way. I think this occurs because the .off() method of jQuery doesn't work in IE in the same way as in the other browsers.

Could you please read my comments on the commits, give feedback on them and, if you like them, pull my changes? (and figure out how to fix this bug, maybe?)

Thank you very much for the plugin!

PS: I didn't test multi-selection nor list mode, because we only use the default mode here.