Add support for file info records (#33)

I needed file modification dates so I could serve assets over go's HTTP server reliably. This implements #33.

Basically I added another function `AssetInfo` that's a lot like `Asset`. There's a chance of breaking backwards compatibility, but only if someone already had a function named `AssetInfo` in their code.

Not sure if this is what you want for a final version, but maybe it can be a starting place.