Closes #641

Removes use of getElementsByClassName

And fix test suite which was demanding the wrong result.

Closes #410

Closes #342

Closes #307

Since v2 is the canonical CodeMirror at this point

It's tiny anyway, no need to force people to include yet another file.

This is quicker to update, and allows querying both by height and by
line number, which will help with the text-wrapping and code-folding
implementations.

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).

This might break some client code.

There is now a CodeMirror-scroll class that is used to provide the
scrolling, rather than using the outer wrapper for this. This allows
adding fat borders to the wrapper without causing glitches in
cursor and mouse-click positioning.