you know you're just gonna merge it... BEM + styleguide-diff + fixes

- `content` is no longer improperly quoted if not needed
- dropped `gradient` from supported keywords in styleguide components (use `background-image` instead)
- added `$CONFIG_STATE_BEM` and `$CONFIG_STATE_BEM_SEPARATOR` to generate [BEM style state selectors](http://bem.info/method/)
- `$CORE_STATE_MAPPINGS` and `$CONFIG_STATE_MAPPINGS` are now proper Sass maps
- `styleguide` function now has the same signature as the mixin and can take multiple definitions at once
- `styleguide-diff` now takes two `styleguide` results as params