Add in-field labels to login form

[gsaines]

Karen suggested that we add in-field labels to the login form to be consistent with other forms on the site. Although I think there is a convincing argument to be made against using in-field labels at all, I think it's more important to be consistent and the sign up form makes heavy use of them. If possible, it would be great to keep the in-field labels around even when the field is selected (like Twitter does) and only remove the label when the user starts typing.