Cache invalidation for assets from pipeline

[FredrikNoren]

Currently you have to manually delete tmp/http to clear the cache when you make a change to any asset or the pipeline importing it. We need to figure out a way to automate this.

One idea would be to hash the content of assets and put them under a sub-dir based on the hash.

What we come up with also needs to work for hot reloading (#12 )

[philpax]

Using hashes is pretty standard I think - I would need to check but I'm like 99% sure that's what GMod does with Steam Workshop

[FredrikNoren]

I've disabled caching for now (and made it configurable)