Responsive

Here's a first draft — (but it fixes a lot of wonkiness and mobile bugs, and kills a bit of complexity, so I think it's worth merging) — of responsive document viewers.

New API options: `{responsive: true}`, and `{responsiveOffset: 100}`.

The `responsiveOffset` defaults to 100, but you can override it. You'd use it to determine how much smaller you'd like your document to be than the vertical viewport size. For example, large fixed headers like the New Yorker might want 150.

Kick the tires on the test page that has the container CSS'd with a few different breakpoints: viewer-responsive.html