Expect muck to automatically create _build directory

[gabestein]

If you start a muck project without a pre-made _build directory, it will give you the error:

FileNotFoundError: [Errno 2] No such file or directory: '_build/_muck_info.json'

This can be resolved by manually creating a _build directory at the project root. Would be somewhat more user friendly to automatically create the directory if it doesn't exist.

[gwk]

Resolved.