Send a 201 Created header with new attachments

See #272

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2283 f69abe3c-2ee2-4550-9a6d-17bd458115be
Split the post-related routes into a separate document

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2284 f69abe3c-2ee2-4550-9a6d-17bd458115be
Add documentation on using media routes

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2285 f69abe3c-2ee2-4550-9a6d-17bd458115be