Allow receiving sender in method call

Exported methods can receive the sender of the
message by specifying an argument of type
dbus.Sender. Upon calling, this argument will
be set to the sender of the message. Any
dbus.Sender arguments are not part of the dbus
method signature.
Move Sender type to export.go
Document sender type
Document new export behaviour for Sender type
Merge pull request #41 from jessevdk/issue/40

Allow receiving sender in method call