Made dbus-sharp run properly on Win32.

To achieve that, these changes were necessary:
(a) The static instances of the system and session bus were hardcoded to unix: transport, which led to an exception due to missing libc on Windows.
    For now, I set their initialization to null on Win32.
(b) Disabled the unix: transport on Win32 at runtime, so no exceptions for missing libc occur anymore.
(c) Added missing Authenticate() implementation for tcp: transport (libdbus requires sending the Windows user ID there)
(d) polished the *.csproj a little
(e) repaired unmanaged/managed buffer copying code

I verified that this implementation now successfully runs with official
dbus 1.7.8
from
http://dbus.freedesktop.org/releases/dbus/