mdButton: default min-width excessive for small buttons

[ngraef]

A button's min-width added in a936e1e is too wide in certain use cases, including:

1. A non-fab icon button (demo):
   
2. A short text button in an action toast (demo):
   

[marcysutton]

I think small or icon buttons not in a Toast (or other specific component) will need a special class, since there is nothing differentiating them from flat and raised buttons (which require the min-width).

[marcysutton]

This can be outlined in Toolbar + icon demos #1796