Include an affinity token by default as GET `a`

An additional token should now be sent with
every request as GET variable `a` by default.

Per #35 a unique token should be created per instance of
BCSocket, allowing a correctly configured load balancer to
consistently route all requests from the same BCSocket instance
to the same Browserchannel server.
simplify affinity options per #37
explain session affinity options, closes #35
[closure-library#df47692] compile c6ed2f2
Merge branch 'feature/affinity'