Better error messages in nested less files
Raise an error when a manifest file is missing
Add test.
Very large refactor.

- make everything user requires, instead of uses
- qualify all names
- all assets to register themselves
- move some functions from core into mime.clj and precompile.clj
Move cache stuff into cache.clj
Move md5 functions
Dont pass options in directly to assets
Port some path functions to URI functions
Use higher-level function
Asset-builder doesn't need options
Actually, the asset builder doesn't need to be an entry point at all
Remove a "path" usage
Introduce the "Asset relative filename", and start using it as a canonical path representation
Make that "Asset directory relative filename"
Start breaking things
Move functions to different section
Fix variable name
Rename uncachify-* to uncachify-path
Improve asset-roots comment in README
Refactor compression conditional