Material-ui: [FontIcon] Port component

[mbrookes]

• [ ] Component
• [ ] Tests (at least unit tests)
• [ ] Docs
• [ ] Demo

[mbrookes]

@rosskevin has posted an implementation: https://github.com/callemall/material-ui/issues/4794#issuecomment-247430489, just needs docs and tests.

[rosskevin]

I'll take this and PR it. I propose we simply shorten this name to Icon. Acceptable? I don't think FontIcon is needed to distinguish it from SvgIcon.

• [x] Component
• [x] Tests (at least unit tests)
• [x] Convert current code span class='material-icons' to use Icon
• [x] Add regression tests
• [x] Regenerate baseline
• [x] Docs
• [x] Demo