[WIP Preview] New secure hierarchical deterministic wallet address manager.

This pull request is a work in progress and is being opened as a preview.  It implements a new secure, scalable, hierarchical deterministic wallet address manager.

The following bullets are an overview of what is already supported:
- BIP0032 hierarchical deterministic keys
- BIP0043/BIP0044 multi-account hierarchy
- Strong focus on security:
  - Fully encrypted database including **public** information such as addresses as well as private information such as private keys and scripts needed to redeem pay-to-script-hash transactions
  - Hardened against memory scraping through the use of actively clearing private material from 
memory when locked
  - Different crypto keys used for public, private, and script data
  - Ability for different passphrases for public and private  data
  - Scrypt-based key derivation
  - NaCl-based secretbox cryptography (XSalsa20 and Poly1305)
- Scalable design
  - Multi-tier key design to allow instant password changes regardless of the number of addresses stored
  - Tests have been performed against a sample database with 1 million addresses in it with no degradation in performance
  -  It currently takes about 1.5 seconds per 1000 new addresses generated on commodity hardware
- Import WIF keys
- Import pay-to-script-hash scripts for things such as multi-signature transactions
- Ability to export a watching-only version which does not contain any private key material
- Programmatically detectable errors, including encapsulation of errors from packages it relies on

Things left before it's ready for merge:
- Implement address synchronization capabilities
- Add full test coverage
- Add examples
- Add README.md
Fair point.  That starts to rely on implementation details such that if it ever changes, we'd be zeroing the slice out from under the big.Int, but that's easy enough to add a test for to ensure it doesn't happen.