SSR Support

[brianjd]

Not exactly the same issue as #62 , so I created a new issue.

I am also having difficulty with getting SSR to work with styled-jss. Using createStyled, I create a new styled using an existing jss instance. Though the styles render client-side, they aren't making it into the registry, and therefore not getting injected into the critical CSS. Is the jss instance supposed to handle the insertion of styles into the registry? Or this is something styled-jss is supposed to handle that hasn't been implemented yet?

[kof]

SheetsRegistry is provided by JSS, but adding to it is a task of the react integration.