Add support for VaultUser and VaultGroup permissions resources

[davecap]

The client currently supports the Vault resource but does not allow easy maintenance of permissions. The API has the following resources:

• VaultUser (/v2/vaults//users)
• VaultGroup (/v2/vaults//users)
• VaultCollaborator (/v2/vaults//collaborators)
  • A read-only view that shows all users with access to a vault, as well as their permission level

Proposal: add the following methods:

• Vault.users() -> returns a list of VaultUser objects
• Vault.add_user(user_id, role='read')
• Vault.groups() -> returns a list of VaultGroup objects
• Vault.add_group(group_id, role='read')
• Vault.collaborators() -> returns a list of VaultCollaborator objects

[jsh2134]

This was requested today for use in syncing permissions to a third-party system (specifically NGB)