Allow WP XML-RPC Library to be Extended

[filosofo]

WP defines a large set (thousands of lines of code) of XML-RPC server methods in xmlrpc.php.

Unfortunately, WP's server class is defined, instantiated, and executed all in the same file, which makes it difficult to use those methods elsewhere (for example, in my JSON RPC plugin).

My patch moves the XML-RPC class to a file in wp-includes.

[westi]

[15647][15648]
Leaving to soak for a bit.
May need more tweaks

[hakre]

move include statement for IXR as well

[hakre]

Most probably for the admin include as well

[hakre]

I guess those were missing :)