Log Message:
Update to 1.4.18

Upstream changes:
27 September 2013: statmod 1.4.18
- Update reference for permp().
- bug fix to elda() so that it returns NA for the tests instead of
  giving an error when the Fisher information for the slope isNA.
- Exact roles of authors specified in DESCRIPTION file.
- All usage lines in help files wrapped at 90 characters to ensure
  that code is not truncated in pdf manual.