Added --unminified option to build.

This adds an option to the build script to create an unminified version of Easel. It's really nice to have a readable version during development that we can actually step through in the JS debugger, without having to include all of Easel's individual files.

This implementation relies on cat, and thus only works on unix.

Don't know if you want this in the main project, but I found it useful….