WIP: Accessible Tabs

To be accessible from a keyboard and screen reader, `material-tabs`, `material-tab` and associated content panels require ARIA attributes and `tabIndex`. The expected behavior for a tab panel is to navigate with the keyboard onto the current tab, and then navigate to neighboring tabs using the arrow keys. `tabIndex` have a value of `0` on the current tab and `-1` on other tabs. `aria-selected` is dynamically updated along with `tabIndex` to communicate the currently selected tab to non-visual users.

TODO: add `role="tabpanel"` to tab panel content areas and associate them with their corresponding tabs with `aria-labelledby` and `aria-owns` on the tab.

For more information about accessible tabs: http://www.w3.org/TR/wai-aria/roles#tablist and http://www.marcozehe.de/2013/02/02/advanced-aria-tip-1-tabs-in-web-apps/