v0.2.0

BACKWARDS COMPATIBILITY NOTICE:

• Requires Vault 0.1.2 or later for X-Vault-Token header
• auth_token method removed in favor of token property
• read method no longer raises hvac.exceptions.InvalidPath on nonexistent paths

IMPROVEMENTS:

• Tolerate falsey URL in client constructor
• Add ability to auth without changing to new token
• Add is_authenticated convenience method
• Return None when reading nonexistent path