openpgp_keyring include subkeys by ID

openpgp_keyring.getPublicKeysForKeyId will now search subkeys
when looking for the ID, and return a key object based on that
subkey.

Updated to add a method, getSubKeyAsKey, to openpgp_msg_publickey.
This returns a full-fledged openpgp_msg_publickey object for a
chosen subkey.
minimize.sh make file ordering deterministic

Do find | sort | cat instead of find -exec cat when building
resources/openpgp.js. The order of files returned by find isn't
deterministic, which could lead to spurious differences.