Work on orientation for tabs

[ZoeBijl]

As discussed during telecon.

Also, when working on this, fix up/down arrow key problem described in issue #14.

[mcking65]

This issue is addressed in commit 424776b and can now be viewed in master at:
http://w3c.github.io/aria-practices/#tabpanel

Feedback is welcome!

[mcking65]

Change 2nd bullet:
"When focus is on a tab element: "
To
"When focus is on a tab element in a horizontal tab list: "
And then separate the non-arrow key bullets out from that sublist.

[mcking65]

This issue is now fully resolved with the addition of commit aeab2e1