New option to run from D-Bus session bus.

The generated client code will also connect to the session bus if the
PD_USE_SESSION_BUS environment variable is set.
Test harness, with first real test case.

To run tests, a new D-Bus session bus is launched (or, on hosts without
dbus-run-session, the existing session bus is used) and printerd is run
on that bus. It remains running until the final test,
stop-session-service, is run.

The actual test case, tests/version/run-run, checks the Version property
of the Manager object.
New test case: create and delete a printer.

Expected to fail at the moment due to a GLib-CRITICAL message in output.
Revert "Don't use deprecated g_type_init()."

This reverts commit 9f505cb05279a4961c8624a306f17ef1b15f44d7.

The g_type_init() call is still needed when building against older
versions of glib2.

Found with Travis CI.
Fixed message about removing unknown source when deleting printer.

This fixes createprinter1 test case.
Fixed default document-format attribute.

Now createprinter1 passes.
New test case: createprinter2.
New test case: createprinter3.

This creates a printer and inspects its properties.

Currently expected to fail.
New test case: createprinter4.

This tests Manager.GetPrinters().
New test case: job1.

This creates and cancels a job, inspecting its properties.
Merge pull request #28 from twaugh/tests

Merged test suite.