Fix: Added token to params for sessions.

[abhisekpadhi]

Tokens in params was missing when creating and manipulating consul.Sessions objects. Since the consul.Consul object already has token in attributes, I'm trying to refer to the agent's token attribute via self.agent.token.

cc: @matusvalo @abn @cablehead can you please review.

[timp87]

See also https://github.com/cablehead/python-consul/pull/238