Add vendor/.bundle to .gitignore

Doing a bundle install --path vendor adds some directories
we don't need to commit. Installing this way keeps your installed
gems to a minimum.
Fixes #50: use knife vault for all commands

Based on the request for issue #50, this replaces the knife encrypt
and knife decrypt commands with knife vault.

This follows the convention of knife where you operate on an object with
commands (knife node edit, knife user edit, etc)

This also makes more sense because you're not creating an "encrypt" but
actually a "vault".
Fix JSON in KNIFE_EXAMPLES.md
Add VaultBase for common code extraction

All vault commands now include the VaultBase
module which contains the common dependencies and
mode option.

The commands have also been updated to fit the
class Chef
  class Knife
    class NewCommand < Knife
definition pattern.
Use knife[:mode] if defined in knife.rb

Inside of the knife.rb file, add the line:
knife[:mode] = 'client'
to default to client mode. If the value is defined
in the knife.rb file, it can still be overridden
by the command line argument.
Add back deprecated 'encrypt', 'decrypt' commands
Add rotate all keys command

This will rotate keys for all existing vaults
and items.
Update KNIFE_EXAMPLES.md, README.md

Fix global options to include subcommands that
take the option
Add documentation for rotate all keys
Explain how to override default mode by modifying
knife.rb file
Proper use of JSON
Use knife[:vault_mode] for knife.rb file
Rename the 'mode' option :vault_mode

This solves the issue of using vault_mode in the
knife.rb file and still allows users to continue
to use the -M and --mode command line flags.
Merge commit '2da49f3' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Resolve merge conflict
Merge commit 'a8db4ab' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit 'c083ba8' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit '1829afd' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit 'b43bf7a' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit '7c43429' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit 'f8eaae7' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit '98ec032' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys
Merge commit 'cadae17' into add_vault_commands_deprecate_encrypt_add_rotate_all_keys