Implement hdkeychain BIP0032 API.

This commit adds a new sub-package named hdkeychain which can be used to
derive hierarchical deterministic key chains which form the foundation of
hd wallets.

- Support for private and public extended keys
- Convenient cryptographically secure seed generation
- Simple creation of master nodes
- Support for multi-layer derivation
- Easy serialization and deserialization for both private and public
  extended keys
- Support for custom networks by registering them with btcnet
- Obtaining the underlying EC pubkeys, EC privkeys, and associated bitcoin addresses
  ties in seamlessly with existing btcec and btcutil types which provide
  powerful tools for working with them to do things like sign transactions
  and generate payment scripts
- Makes use of the btcec package which is highly optimized for secp256k1
- Code examples including:
  - Generating a cryptographically secure random seed and deriving a
    master node from it
  - Default HD wallet layout as described by BIP0032
  - Audits use case as described by BIP0032
- Comprehensive test coverage including the BIP0032 test vectors
- Benchmarks
Correct example links in hdkeychain READEME.md.
Correct DefaultWalletLayout example link.
Add new function on extended keys to zero them.

This commit adds a new function named Zero on the hdkeychain.ExtendedKey
which can be used to manually clear the memory used for an extended key.
This is useful for enhanced security by allowing the caller to explicitly
clear the memory when they're done with a key.  Otherwise it might hang
around in memory for a while.

Once a key has been zeroed it is no longer usable.

This commit also contains tests to ensure everything works as expected
after a key has been zeroed.
Test variable declaration consistency.
Update hdkeychain test coverage report.
Fix formatting directives in tests.

Found by 'go vet'
Add bloom filter Reload method.
Made App Engine runtime compatible.
Remove embedded bloom.Filter mutex.

This prevents the caller from being able to accidentally lock or
unlock access to the filter internal state.

While here, remove several defers that do not gain us any readability,
and only hurt our performance.
Update test coverage report.