trunk-sync: refactor SystemRewriteOptions, psuedo-RTTI removed

This is the ngx_pagespeed half of a refactor morlovich made:

    Split out the settings that can be shared between
    mod_pagespeed and ngx_pagespeed into a separate class.

    This is step 1 out of ??? of refactors to reduce code dupe
    between mod_pagespeed and ngx_pagespeed and make some
    things more testable.

    (Step 2 will be to share some of the configuration parsing
    code).
     -- https://code.google.com/p/modpagespeed/source/detail?r=2443