Create users via XML-RPC

[nprasath002]

enables admin users to create users via xmlrpc.
only users with capability 'create_users' can create a user.
Users cannot register themselves because it will allow spammers to create mass accounts.

[maxcutler]

Cleaned up patch formatting and error codes.

Refined validation logic to remove some duplication of code from wp_insert_user (which will be caught by is_wp_error at end of the method).

Refined send_mail logic to reuse existing pluggable functionality.

[nprasath002]

Im not sure about having two parameter names (user_login and username).
I first developed as this and changed it following a discussion
in GSoC blog and with my mentor.
​http://gsoc2011.wordpress.com/2011/07/23/wp-web-services-%E2%80%93-weekly-update/

Anyway its not a major issue

[nacin]

Per IRC discussion (see #18428), this isn't going to be part of XML-RPC for the time being.