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 keyboard interaction for a tab panel is to navigate to the current tab using the tab key, then to neighboring tabs with the arrow keys. The current tab has `tabIndex="0"` and the inactive tabs have `-1`, so only the current one is in the tab order. For assistive technologies, `aria-selected` is dynamically updated to communicate the currently selected tab to non-visual or low-vision users.

TODO: add `role="tabpanel"` to tab panel content areas and associate them with their corresponding tabs with `aria-labelledby="[tab_id]"` and `aria-owns="[tabpanel_id]"` on the tab.

For more information about accessible tabs: 
- http://www.w3.org/TR/wai-aria/roles#tablist
- http://www.marcozehe.de/2013/02/02/advanced-aria-tip-1-tabs-in-web-apps/