Applied accessibility features

Hey there,

I used the chance to add some ARIA-roles and states, so a screenreader should be able to deal with the OWA now, too. The lang-attribute guarantees the correct spelling. Also closed your image tags ;-)

Oh, please, please, don't forget to set explicitely background-color to white and text color to black in all your future apps. It's a mess with a dark themed Firefox on Linux otherwise >_<#

Next issue on my list is keyboard accessibility and localising. Mozilla recommends using the [L10n.js](https://hacks.mozilla.org/2013/08/localizing-firefox-os-apps/). Have a look into it.

I glanced the current issues and wonder, whether there's a way to determine the login state of an user? Afaik Geraspora has not published an API, yet.