gnupg-for-java: also allow fetching secret keys using GnuPGKey contructor

Before, it would only search and return public keys
gnupg-for-java: allow fetching secret keys using GnuPGContext.getKeyByFingerprint()
gnupg-for-java: add listSecretKeys() and searchSecretKeys() to GnuPGContext
make APG's SELECT_SECRET_KEY Intent only show secret keys
gnupg-for-java: simplify and complete get/set for Armor and TextMode
gnupg-for-java: add GnuPGData.size() to improve ByteArrayOutputStream performance

Since ByteArrayOutputStreams are used to get data out of GnuPGData, add a
size() method so that the ByteArrayOutputStream can pre-allocate the memory
needed, or at least close to the final number.
gnupg-for-java: add javaish encryption methods
working implementation of encrypt-only ENCRYPT_AND_RETURN APG Intent