Allow --hidden-recipient to be used for a list of recipients.
Actually add test to be run and format variable correctly.
Update to allow multiple keys to be shown on list_packets.
Also updated test to ensure only expected keyid has been hidden.
Add parameter description
Merge remote-tracking branch 'tomgalloway/develop' into fix/67-hidden-encrypt
Add throw_keyids=bool option to encrypt() method.

 * FIXES part of Issue #67.
   https://github.com/isislovecruft/python-gnupg/issues/67
Change ListPackets.key to be backwards compatible.

This also adds a new ``ListPackets.encrypted_to`` list, which contains
all the keyids which a message was encrypted to. ``ListPackets.key`` is
just the first one we discovered that it was encrypted to, for backwards
compatibility.

 * FIXES part of Issue #67.
   https://github.com/isislovecruft/python-gnupg/issues/67

 * CHANGES the additions made in commit f70a7dc4f09 by @tomgalloway in
   PR#70 to be backwards compatible.
   https://github.com/tomgalloway/python-gnupg/commit/f70a7dc4f0968d8abad900db285394b71adb5f2e
   https://github.com/isislovecruft/python-gnupg/pull/77
Merge branch 'fix/67-hidden-encrypt' into develop