Added chef-client support via shared ChefCommandBuilder

- Extracted ChefCommandBuilder class out of chef-solo monkey patch
- Backfilled tests for logic in ChefCommandBuilder
- Backfilled tests for is_paralells?
- Added is_windows? instance method to WindowsMachine
- Bumped version to 1.6
Looking to the future, support Ruby 2.1.0 and dropping 1.9.2
1.6.0.pre.1 prerelease gem
Merge remote-tracking branch 'origin/master' into chef-client-support
Bump version to 1.6.0
Merge pull request #178 from WinRb/chef-client-support

Chef client support