Adds support for nested modules, detect circular references.

- Detection of circular references in an attempt to warn users that call
  `coffee` on a diretory which has multiple .coffee files. It's not
  perfect, but works most of the time so is fairly effective as a
  warning mechanism.
- Needless now calls itself on itself (INCEPTION!), to allow nested
  modules to re-require needless without worrying about relative paths.
Bump version to 0.1.0.