Allow passing binary data

[daurnimator]

DBus strings must be valid utf-8

Provide a way to deal with binary data.

[daurnimator]

Binary data can be done with an array of bytes.

It seems reasonable to add an option to append_basic that creates this form.

[ziyao233]

I'm playing with bluez whose DBus API represents sent/received binary data as byte arrays. A native solution is that pack the data with string.pack() and then pass the string (actually binary data) to ldbus. So I'm willing to take a look with this issue.

The idea to add an option to append_basic() doesn't look good to me:

• Currently our bindings match well with the DBus C API documentation, one could figure out how the bindings work easily with it. Such a parameter will break the simplicity.
• The name, append_basic(), doesn't match the new functionality. Anyway, byte array isn't a basic type at all.

I suggest to bind C API dbus_message_iter_append_fixed_array() and dbus_message_iter_get_fixed_array(). We could treat bytearray specially in the binding and convert it to/from Lua strings representing binary data.