Added the ability to set knife_config_file to :auto

When the knife_config_file is set to :auto it will create a simple
knife.rb file with the chef values from the Vagrantfile. The auto
knife.rb will be created in a hidden cache folder waiting for the day
that you destroy the box. Once the box is destroyed the auto knife.rb
will be removed.
Removing unnecessary methods.
Refactoring the word machine to use the word guest instead.

I feel as though the word guest is more clear when referring to the
guest machine.
Adding two requirements to the README.

Also added a warning if the /vagrant folder isn't mounted.
Merge pull request #12 from flowboard/auto-knife

Added the ability to set knife_config_file to :auto