dbus: Set GObject event loop as default DBus loop

This must be done early in the process life cycle to have any effect. In the
MPRIS frontend code is too late. Right before the GObject loop is created in
RootCommand.run() is also too late.

Fixes mopidy/mopidy-mpris#2
docs: Update changelog
tests: Don't redefine PYTHONPATH

Shot in the dark to get Travis to find dbus-python when running the `mopidy`
command as a subprocess of the tests.
travis: Manually install python-dbus

Now used by GObject eventloop registration, Zeroconf integration, and keyring
integration.
Revert "tests: Don't redefine PYTHONPATH"

This reverts commit 6be5cc0f1b1d34521fa8d8c91ca1cc2a96a65b69.
Revert "travis: Manually install python-dbus"

This reverts commit 16a76d4ea5a08c09acc7337cd56b431c3f9f376c.
dbus: Skip mainloop setup if dbus isn't installed
config: Add support for formating with commented out defaults

- Changes inline comments to ; for configparser conformance
config: Add format_initial helper to populate empty configs.
utils: Add mkdir and content options to get_or_create_file

This lets callers control is parent directories are created, and set the
default content of the file.
config: Initialize config with defaults

Try and create args.config_file[-1] with the default config commented out.
We assume that the directory the config file is in exists.
config: Fix review comments.
Merge pull request #588 from adamcik/feature/config-defaults

Initialize config with commented out defaults.
docs: Update changelog
docs: Update changelog for v0.17.0
Bump version to 0.17.0
Release v0.17.0