Show register and sign in buttons (bug 1072619)

https://bugzilla.mozilla.org/show_bug.cgi?id=1072619

![screen shot 2014-10-08 at 16 50 16](https://cloud.githubusercontent.com/assets/211578/4567872/367f94fa-4f35-11e4-9dc6-2cacf0ab9a36.png)
1140px wide

![screen shot 2014-10-08 at 16 50 42](https://cloud.githubusercontent.com/assets/211578/4567873/369a8e2c-4f35-11e4-95c7-4563575a7804.png)
768px wide

![screen shot 2014-10-08 at 16 50 55](https://cloud.githubusercontent.com/assets/211578/4567874/369a9d22-4f35-11e4-9bb5-3503614b1948.png)
320px wide

we've been calling it "Sign in" and "Sign up." If we call this "Register," why don't we call it "Log in" instead of "Sign in"?

FWIW, FxA calls it "sign up" — not "register."