Better SSR methods

[rauchg]

Fixes #22

• Adds styled-jsx/server
  • flush() (default) returns an array of React.Element for each <style>
  • flushToHTML returns an HTML string

Additionally

• Both new methods include an id in the resulting <style> that the client rendering part can use to de-dupe.
• The de-duping logic only checks the DOM once and works transparently, without manual initialization by the user

[nkzawa]

It looks we have to fix styles (CI is failing)