Update appcache per-build & untrack.

Untracking built app cache.
Revision is now a number.
SHA and Date for reference.

May it be said that I despise node's way of doing file read / write operations.

I might end up moving the git SHA stuff into it's own module. Seems useful enough.

Refers to #37 and https://github.com/artursapek/mondrian/issues/13#issuecomment-34120230.