Problems with signup screen

[aaronaverill]

Currently the signup screen uses the user/add controller, which is fine, but it needs some refinement.

1. The Is Admin checkbox should be hidden
2. The "Create User" button should be labeled "Sign Up"
3. The "New User" heading should say "Welcome!"
4. The Cancel link should be hidden
5. After signup, it should log the user in and redirect to posts