more doc changes; renamed features

- change "docs" to be the default target in doc/Makefile.
- add Stephen the list of authors in cffi-manual
- rename "Index" to "Comprehensive Index" so that Index.html doesn't
  clash with index.html on OSX.
- add varargs example for defcfun
- rename features (again) foreign-funcall -> no-foreign-funcall,
  long-long -> no-long-long.