Make default scroll style for CodeMirror-scroller 'scroll'

And manually compensate for our own scrollbar sizes.

This way, we're no longer dependent on the size of our scrollbars
being the same as the size of native scrollbars, and can start to
think about custom scrollbars.
Add some scroll model tests
Add the scrollbarStyle option

Implements a pluggable scrollbar model. Interface not documented yet,
but quite simple (see forthcoming simplescrollbars addon)
[simplescrollbars addon] Add
Expose CodeMirror.wheelEventPixels