Add knife vault mock, update documentation

Add knife vault mock command for test kitchen use

We've added a command to create mock vaults for use with vagrant and test-kitchen.

    knife vault mock VAULT ITEM VALUES --vm NODE_NAME

The code will write out a directory named VAULT containing two files, ITEM.json and ITEM_keys.json.

The ITEM_keys.json includes a new hash key "secret" which will allow the installed gem to decrypt the encrypted ITEM.json file using the "secret" key.

You need to move the VAULT directory into your test/integration/default/data_bags directory where the Chef server will make it available.

Explain how to use knife vault mock and clean up a few differences between the KNIFE_EXAMPLES and README command option tables.

Fixes #61