make FlexSlider (more) accessible to screen readers

I've made some modifications to FlexSlider to make it (more) accessible to users who use screen readers and / or use the keyboard to navigate. I think it would benefit the community at large if you would integrate these modifications into the code base.

Notes on the code: To properly set the control's aria-controls attribute, the slides need to have a unique id attribute. Where the id attribute is already present, I use it. Where it is not present I generate one. To handle the case where there are multiple sliders on a page I use / generate a unique id for each slider, then use that to generate id's for the slides. Note that when flexslider() is called multiple times on a page on elements which do not already have id's, the generated id's may not be unique. However I feel this is a relatively rare case, and there's an easy workaround - specify unique id's for each slider, so I feel it's acceptable behavior.

Thanks for your consideration.