Add encrypted field support to Python SDK

In support of Field-Level Encryption (sdk-rfc #32, draft)

Implement common approaches to:

• key management
• encryption of a field/fields

Note that this is, at this time, client side operation only - no server-side access to encrypted data required.  This addresses use cases where fields may be accessed, but aren't queried by range or otherwise processed by query, fts, etc.