Simple agent_client cli

Supports ping/state requests. Useful for bash/bats integration testing of bosh_agent
to validate the agent is up and running.

Examples running on a bosh_agent VM:

    $ agent_client -m https://vcap:vcap@localhost:6969 ping
    pong
    $ agent_client ping
    pong
    $ agent_client state
    {"deployment"=>"", "job"=>"", "index"=>"", "networks"=>{}, "resource_pool"=>{}, "packages"=>{}, "persistent_disk"=>{}, "configuration_hash"=>{}, "properties"=>{}, "agent_id"=>"micro", "vm"=>nil, "job_state"=>"running", "bosh_protocol"=>"1", "ntp"=>{"message"=>"file missing"}}

And for incorrect https agent URI:

    $ agent_client -m https://vcap:vcap@localhost:9999 ping
    /mnt/hgfs/!%bosh/agent_client/lib/agent_client/http_client.rb:46:in `rescue in request': Request details: (Bosh::Agent::Error)
    uri: https://localhost:9999/agent
    payload: {"method":"ping","arguments":[],"reply_to":"Bosh::Agent::HTTPClient"}
    user: vcap
    password: vcap
    Errno::ECONNREFUSED: Connection refused - connect(2) (https://localhost:9999)