SSR Support

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?

Start by reproducing SSR with createStyled and an existing JSS instance, then trace why client-rendered styles are not entering the registry or critical CSS. Done means the styles are registered and included in the generated critical CSS during SSR, with the behavior verified in an appropriate test.

Written by the indexing model from the issue text.