Are we ready for a final API?

This version is a preparation for v0.9.0, which is going to bring API changes, as described in #127.

• new functions in Drab.Element (#134, #135)
• new Drab.Coder for encoding terms to string (#137)
• new js_socket_constructor config (#133), useful when using Drab with Webpack
• all assigns are now peekable (#126)
• broadcast_poke now gets subject, not only socket (under some limitations) (#141)
• preserve csrf token if poke updates the form (fix for #130)