Add concept of sub-domains

[nhunzaker]

I want to slowly move towards Domains just being Microcosms mounted at paths to other Microcosms:

import Domain from 'microcosm/domain'

class Node {
  getInitialState() { return [] }
}

class Edge {
  getInitialState() { return [] }
}

class Network extends Domain {
  setup () {
    this.addDomain('nodes', Nodes)
    this.addDomain('edges', Edges)
  }
}

const repo = new Microcosm()

repo.addDomain('network', Network)

expect(repo.state.network.edges).toEqual([])
expect(repo.state.network.nodes).toEqual([])

I also updated error messages to correctly encounter non-function keys (and respond with null):

Unable to register `action.done` at domain path `["network", "nodes"]`. Handlers must be
functions, instead got `undefined`. Check the register method for this domain.

[coveralls]

Coverage remained the same at 100.0% when pulling 264e5ed on sub-domains into 65f952c on master.

[nhunzaker]

Closing this out after I mused over it a while. See #184