Bug 678338 - Check return values from Python API

https://bugzilla.gnome.org/show_bug.cgi?id=678338
Fix various deprecation warning from GTK+
Disable GLib diagnostics in tests
Add easy python profiling support

When the PEAS_PYTHON_PROFILE enviroment variable
is set profiling information will be output when all python
plugins are unloaded.
Use the buildir when running the demo's in running in builddir mode

Otherwise it assumes the current directory is libpeas/peas-demo
which can be a pain to figure out.