Check the group membership using dscl on Mac.
Merge pull request #1973 from opscode/sersut/group-spec-fix

Check the group membership using dscl on Mac in specs.
Extract Knife config file find/load to a reusable component
Knife now prefers DIR/config.rb to DIR/knife.rb

`knife.rb` is not deprecated at this time.

The precedence of directories is the same. Directories are searched in
the same order, so that the presence of config.rb in a low precedence
directory, e.g., `~/.chef/config.rb` does not prevent knife from loading
a knife.rb located in a higer precedence directory, e.g.,
`$PWD/.chef/knife.rb`.
Remove config file jail, replaced with config skip option
rename '--dont-use-config' to '--disable-config'

A little clearer/less verbose
Add debug/info logging for knife config file finding

To resolve a chicken/egg issue with the logger being configured *after*
the config is located and read, knife will now check for a KNIFE_DEBUG
variable and initialize the logger to debug early in the startup process
when that variable is set.
Update doc files for knife `config.rb` support.