dropdown-menu prototype, item code

Adds .DropdownMenu and .PaperItem to help resolve #475.

.PaperItem is a button-like object which has no outline - similar to [paper-item](https://www.polymer-project.org/components/paper-elements/demo.html#paper-item).
* .PaperItem (and its css/js) is a bit redundant vis-à-vis .PaperButton, opinions?
* Hover/active/ripple colors are absolute, but .PaperItem should be able to be put above any color: so keeping it separate would allow me to change these to be transparent just for .PaperItem.

.DropdownMenu is just a holder for now, but I'll expand on it to be 'attached' to things in a following change.