Move the test helper to a subdirectory

This stops it from appearing in the plugins list.
Corrected parameter names in x-form examples.
Merge pull request #127 from WP-API/move-testhelper

Move the test helper to a subdirectory
Include post ID with json_prepare_meta filter

Props @kuching. Fixes #67.
Merge pull request #137 from WP-API/prepare-meta-filter-post-id

Include post ID with json_prepare_meta filter
Merge pull request #134 from rachelbaker/docs-updates

Corrected parameter names in x-form examples in docs.
Pass the server instance to json_serve_request
Add hook documentation
Merge pull request #139 from WP-API/add-serve-request-arg

Pass WP_JSON_Server instance to json_serve_request.  Fixes #61.
Add show_in_json argument to register_post_type