Use file checksum for cacheBuster

[jfexyz]

It might be nice to switch to using a generated hash for the cacheBuster, so that the cacheBuster is only updated when the actual file contents change, not every time bless is run.

A small CSS change could theoretically result in identical blessed files, and it'd be nice to not require users to re-download them.

[paulyoung]

Good suggestion.

[davidwindell]

+1

[paulyoung]

Closing since this is being tracked in #57.