Add domain sharding

[GoogleCodeExporter]

Provide directive to allow rewritten resources to be distributed across a set 
of domains:

From gorans on Issue 56:

It would be good to be able to specify the static media domain in a format such 
as: {m1|m2|m3}.staticserve.com and have PageSpeed use an algorithm to split the 
resources between these servers.  This would enable concurrent downloads of the 
media files across the different domains.

Original issue reported on code.google.com by jmara...@google.com on 15 Nov 2010 at 12:14