richMode: Selection.restore -> Selection.get
deps: serialize-elem@0.5.0
richMode: make everything space-aware

Also fixes #48 (see line 45 of enterKey.js).
deps: choice@0.2.0
richMode/selection: normalize less often

This commit actually fixes two issues.

The first issue was that we were normalizing after every keydown, which
was causing issues when, for example, entering accented characters via
Alt shortcuts on a Mac.

The second issue involved tabbing into the editor in Firefox; the caret
is placed at the very start of the element (before even any
<section>s), and Choice was interpreting this as the selection on the
<hr>; that sucked. The <hr>s were made non-contenteditable, and Choice
was updated to ignore non-contenteditable elements.
richMode/spacebar: listen for keypress

This avoids us having to manually run the rich text formatter.
selection: emit ‘selectionchange’ event on restore
Remove unused code.
richMode/view: trim trailing <br>s when syncing.
richMode/backspace: fix an issue with <br>s.