Allow passing Content-MD5 headers

This is only global and cannot be attached to the part in a multipart request.
I do love PHP some times.

See #272

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2272 f69abe3c-2ee2-4550-9a6d-17bd458115be
Add PHPDoc for new attachment methods

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2273 f69abe3c-2ee2-4550-9a6d-17bd458115be
Check create_posts cap before upload

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2274 f69abe3c-2ee2-4550-9a6d-17bd458115be
Return the attachment's data after creation

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2275 f69abe3c-2ee2-4550-9a6d-17bd458115be