Add designated auth credential initializer

Also modify description accessor.
Add initial auth credential storage api

Includes integrating with authenticator.

Includes (tentatively) linking to Security
framework.
Add initial auth credential storage tests

Update authenticated initialization test.

Update shared client setup in api method tests to
use an authenticator, and update base test case to
include the shared client setup.
Merge pull request #8 from 3dna/peng/integrate-keychain

Integrate Keychain Services for authentication