squash! simplified PageAgent

Simplified getResourceTree and getResourceContent by using async.waterfall.

Also modified behaviour of the application, we use argv1 file as url for
the top-level frame. This way we can always load resource content from
disk (we don't have to try to load it from V8 first), since getResourceTree
lists only existing files now.

This commit together with the previous one allows users to set breakpoint
in files that are not loaded yet, which is helpful for debugging module-loading
problems.