Add routing table retrieval and updating functions to Go API.

This is needed by the routing table jsPlumb UI. We need at least a method for:

* Retrieving the routing table and channels, routing blocks and endpoints.
* Saving an updated routing table (and validating it).

Example JSON for routing table retrieval:

```javascript
var routing = {
  campaign_id: 'campaign-1',
  channels: [{
    uuid: 'channel-uuid-1',
    tag: ['apposit_sms', '*121#'],
    name: '*121#',
    description: 'Apposit Sms: *121#',
    endpoints: [{uuid: 'endpoint-uuid-1', name: 'default'}]
  }],
  routing_blocks: [{
    uuid: 'routing-block-uuid-1',
    type: 'keyword',
    name: 'keyword-routing-block',
    description: 'Keyword',
    channel_endpoints: [{uuid: 'endpoint-uuid-2', name: 'default'}],
    conversation_endpoints: [{uuid: 'endpoint-uuid-3', name: 'default'}]
  }],
  conversations: [{
    uuid: 'routing-block-uuid-2',
    type: 'bulk-message',
    name: 'bulk-message-1',
    description: 'Some Bulk Message App',
    endpoints: [{uuid: 'endpoint-uuid-4', name: 'default'}]
  }],
  routing_entries: [{
    source: {uuid: 'endpoint-uuid-1'},
    target: {uuid: 'endpoint-uuid-2'}
  }]
};
```