moved PGPError to pgpy.errors because circular-import errors
""
PEP 8
renamed packet to packetblob because packetblob may, in fact, contain more than one packet
abstracted the ASCII parsing (and building) from signature.SignaturePacket to pgp.PGPBlock to prevent code duplication
moved SIGNATURE_MAGIC into new Enum class Magic at Magic.Signature; added magic for PubKey and PrivKey
updated test_PGPSignature to work with code changes