first pass at new vendor command

Add Berkshelf::Berksfile#vendor
Add Berkshelf::Cli#vendor
Update formatters for vendor output
add acceptance tests for vendor command
provide a sensible help message for the vendor cli command
remove explicit check on .nil?
add regression test for #747
use a default path of ./cookbooks when vendoring with no argument
add function documentation to Berksfile#vendor
exit early if no cookbooks to be vendored
remove trailling :: from ::Berkshelf in CLI
raise an exception when vendoring if the target already exists

fixes #726
fix failing features after changing the aruba temp dir
Merge pull request #729 from RiotGames/vendor-command

`berks vendor` command to replace `berks install --path`