refactor to make home values accessible without name appended

- Will be better to have access to home values without name appended in
  order to create a lookup method (upcoming).
- It seems like it works well to have the defaults defined in _home
  rather than in the public methods.
- Changed data_dirs and config_dirs for consistency with _home.
- Checking exists, defined, or "truthiness" of %ENV values all seem
  equivalent to me since an empty string makes no sense as a value.
minor change to croak message
refactor *_dirs for symmetry with *_home

Refactor *_dirs for symmetry with *_home so it will be easier to
implement a lookup method (upcoming).
add lookup_data_file and lookup_config_file based on spec
Merge pull request #5 from nnutter/lookup

Lookup