Major trunk-tracking update (r2618)

Maksim has made a bunch of changes to reduce the amount of duplicate code between ngx_pagespeed and mod_pagespeed.  These changes are now available for us in svn, so we should update.

After this change I'm planning to:

 - generate binaries for psol as of this svn revision
 - move 'master' to 'stable'
 - move 'trunk-tracking' to 'master'
 - create an new 'trunk-tracking' branch off of 'master'

at which point all new development will be off of 'master' and PSOL r2618.