[nemo-qml-plugin-dbus] Add support to connect to DBus system bus.

Allow specifying which bus to connect to with the busType property.

Fix error reporting by calling lastError() on the correct connection.
Previously the system bus connection was always queried even though
the session bus was being used.
[nemo-qml-plugin-dbus] Allow use of DBus interfaces using object paths and signatures.

Marshalling of arguments was converting object paths and signature
types to strings. The distinct data types need to be used for the
DBus method calls using these types to succeed.
[nemo-qml-plugin-dbus] Add work around for mismatch between DBus and QML method names.

QML method names are restricted to start with a lower case letter. DBus
does not have this restriction. This work around allows DBus interfaces
with methods starting with an upper case letter to be used with
DBusAdaptor.
Merge pull request #3 from amccarthy/fixes

Improvement for DBusInterface and DBusAdaptor QML types