Bump version and API and allow parallel installation

Fix build against latest dbus-sharp from git master by tracking API
changes.

Although there are no API changes in dbus-sharp-glib itself, it
will only work with the dbus-sharp 2.0 API. So it makes sense to make
dbus-sharp-glib parallel-installable with previous versions. So we do
the necessary changes and rename the pkg-config file.

Also update configure.ac to avoid deprecation warnings with recent
autotools.