Rename index.css and require.js with md5 hash

- removes obsolete "cache buster" setting
- replaces hardcoded require.js and index.css filenames in settings
files with for non-dev build processes with variables.
- release and couchapp_deploy tasks now rename those two files to
include their hashes, and reference them in the generated index.html
file
That's true... I actually had that originally, but didn't like having the clean task ran in both of the md5 targets (requireJS + css). Now it just runs a single task to do both. But that said, it didn't require hardcoding the paths like it does now. I'm fine with either approach, to be honest.