Send a payload with POST requests

[crgeary]

Allow POST requests to include a JSON payload. These requests contain a small amount of information about the action that was taken on the WordPress site.

For example, a post/page update might include:

{
  "action": "post_status_transition",
  "data": {
    "id": 1,
    "status": "publish"
  }
}

Solves #40

[nickdbush]

This would be doubly as helpful, as not only would it allow rebuilding the site on publish, but flushing caches as well. Would love to see this soon in the next release.