Model based api

This is still a work in progress, but I put together a set classes that give an accessor-based durable object interface to the tutum api. 

Are you interested in pulling something like this in? If so I'll break this up into discrete pulls and button things up somewhat.

If you have any thoughts on the interface please advise.

```
irb> ncs = Tutum::NodeCluster.list

=> [#<Tutum::NodeCluster:0x007fdcab53eeb0
  @info=
   {:current_num_nodes=>1,
    :disk=>60,
    :name=>"Hadoop",
    ....

irb> my_node = ncs.first.nodes
=> [#<Tutum::Node:0x007fdcab2ca6e8
  @info=
   {:actions=>["/api/v1/action/ffcf03ac-dd2a-4c62-974c-6bd33b8470a4/"],
    :current_num_containers=>4,
   ...

irb> my_node.public_ip
=> "54.69.180.118"

irb> my_svc = Tutum::Service.list.first; my_svc.ports[2]
=> #<Tutum::PortSpec:0x007fdcab2890d0 @inner_port=8042, @outer_port=8042, @port_name="unknown", @protocol="tcp", @published=true>

irb> my_svc.start
...
```