Addition of Checks API to agent

Adds check api to register, deregister and inspect node and service level checks with the agent. The status of TTL based checks can also be set.

I think len(members) should always be 1, as we're running against our own consul instance?

If find assert [x['something meaningful, butreliably'] for x in c.agent.members()] == ['foo'] to be a more reliable type of assertion.