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.