Add in-place resource optimization

[jeffkaufman]

Only html files are currently rewritten on the fly. If a resource can't have it's url modified for some reason, perhaps because it's unsafe to rename, perhaps because it's generated by javascript and not present in the html we're rewriting, then the only opportunity to optimize it is on the fly.

(In some of the Pagespeed code this is called "ajax rewriting".)

[jeffkaufman]

This depends on #102

[jeffkaufman]

Fixed by #450