More bindings for split panes

* added more Vim bindings to focus and close panes
split.md: fix key order so the docs line up nicely
rename splits.md -> windows.md to more closely reference the Vim docs

There are more than just "split" commands in this document now, and
in the Vim docs, all of the `ctrl-w` commands are in `windows.txt`
add missing close binding: ctrl-w ctrl-q
add additional ctrl-key bindings for pane movement
CHANGELOG: added bindings for split pane movement
Merge pull request #162 from atom/more-split-window-bindings

More bindings for split panes