Bug fix: Unmarshal big endian messages correctly.

The endianness of the message was parsed correctly, but the
endianness was not passed to hte unmarshal function, which
defaults to little-endian. This caused big endian data to be
unmarshalled incorrectly.

(Passing on a fix to the official txdbus sources by
Jaakko Korkeaniemi)
Merge pull request #2 from cocagne/master

Bug fix: Unmarshal big endian messages correctly.