MD FlatButton should better conform to MD spec and become the standard for MD buttons in general

Here's what needs to be done to finish this off:
1. Raised button support.
2. Double-halo: quick-fade-in full-sized halo + slow-fade-in-from-touch-point halo (this is the one we already have)
3. Halo larger than hit target: Right now we try to approximate this with padding + margin, but it's laying out lists of buttons too far apart. We need the button element to be the target, and the halos to be sized to be ~105-110% size of button element.
4. Consistent spacing/layout/capitalization for list-of-buttons with/without icon and label.
5. Possibly allow image form of icons to look different when disabled. Right now, "greyed out" icons only work with ligatures.
6. Do not commit actions when mouse/touch leaves then comes back (at least for touch; not sure about mouse).