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.

What do you think about just copying the if statement from here?

https://github.com/cablehead/python-consul/blob/master/consul/base.py#L299

ConsulException kinda means, something went wrong with the call to consul. So raising an AssertionError might be better here - as the call will never be sent to Consul - they've just called the API incorrectly