Add feature to proxy resources whose origin servers don't run mod_pagespeed

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ModPagespeedMapRewriteDomain can only be used to indicate to mod_pagepeed that 
domains are already functionally equivalent.  This can be because the two 
domains are both served by the same Apache server, or because one of the 
domains points to a CDN whose origin points back to the server.

But it is desirable, especially in the context of SPDY, to move foreign 
resources into the origin domain.  A new directive is required to perform that 
mapping.

Original issue reported on code.google.com by jmara...@google.com on 6 Nov 2012 at 8:25