Adding Update function, to PATCH a path

[skeletonz28]

Adding an update function/method, that allows the user to PATCH a secret path This is for instances where a path will have many subkeys. If the user uses the current write method, that removes all previous keys/subkeys PATCH allows you to add on, creating new versions of the path with the added keys/subkeys

Contributing as part of resolving my own logged issue https://github.com/nodevault/node-vault/issues/260