Add grunt-asset-fingerprint (enabled by default)
make assetFingerprint entirely opt-in w/ a flag.
The pages task will now include assets

* An "assets" property with the entire asset manifest is made available
to templates
* The default "css" and "js" context props (from application.coffee) are
overwritten to point to the fingerprinted assets.
* Only does anything of note during "dist" target, but ensures the prop
is available such that the "dev" target doesn't blow up.
Merge pull request #160 from testdouble/fingerprint

Fingerprint