Include normalize using emotion's injectGlobal

[kitten]

Since all our default styling depends on normalize.css being in use (see example code), we could just use injectGlobal to add it.

Optionally there's also a CSS helper in polished to do this.

This would further improve the experience of setting up a spectacle presentation from scratch without a boilerplate.

cc @ryan-roemer @kenwheeler

[kitten]

It doesn't seem like this will be necessary after #399. It turns out that we don't need normalize anymore