Add browser prefixes to CSS3 rules

[akinnee]

Many of the state changes do not appear on iOS because you don't have the webkit prefixes. Example: Focusing on an input does not float the label on iOS event though the mui-is-focused class is added.

[hai-cea]

We actually removed vendor prefixes in v0.5.0 in favor of an autoprefixer. See the notes here:
https://github.com/callemall/material-ui/releases/tag/v0.5.0

Currently, we're using gulp-autoprefixer - https://github.com/sindresorhus/gulp-autoprefixer

Here's how we're doing it:
https://github.com/callemall/material-ui/blob/master/docs/gulp/tasks/less.js#L13

Looks like this is something we need to document.