API: Add prop=contributors

Certain applications, such as the generation of PDFs, could use a list
of all non-anonymous contributors to the page (as well as a count of
anonymous contributors) without crawling the output of prop=revisions.
This patch adds a prop module to retrieve this information.

Including the IP addresses of anonymous contributors is not realistically
possible without further schema changes, so that is not done here.
Additionally, revisions with DELETED_USER will be skipped entirely.

Change-Id: Iaff50dfb09016154901a5197aa14eb9f8febcbc5