Issues 91,114, and 122

Resolves #91
Resolves #114 
Resolves #122 

**Implements the following proposal**: https://github.com/CybOXProject/schemas/wiki/Proposal:-Add-Authentication-Construct-To-Account-Object 

This pull request does the following:
* Adds `Authentication` structure to AccountObjectType
* Creates `AuthenticationType` for capturing the authentication details of an account
* Creates `StructuredAuthenticationMechanismType` extension point for capturing structure authentication information (e.g., fingerprint data)
* Creates CV for authentication types (password, cryptographic key, physical token, etc.)
* Creates CV for methods of protecting authentication tokens (hash, hash+salt, etc.)
* Removes the `Password` field from the `UserAccountObjectType` that was added during 2.1 development