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
Merge pull request #68 from jgeiger/50_fix_command_structure

Fix #51: update knife commands