Add preferences to main app class

[daredoes]

from https://github.com/laffra/happymac

Provides the methods self.preferences.get and self.preferences.set to all rumps apps, established in the home directory if no directory is provided.

The user can set the preferences to the application folder with the following block in the __init__ function of an app, after a call using super

self.preferences = rumps.Preferences(self._name, self._application_support)

[jaredks]

This is cool. I like that it's easier to save data but I wonder now in what circumstance do you use,

app = App('Cool App')
with app.open('data.json') as f:
    json.dump({'key': 'value'}, f)

vs.

app = App('Cool App')
app.preferences.set('key', 'value')

Should these concepts not be consolidated?

Also I think we should use serializer rather than hardcode pickle.

[jaredks]

Suggested change

	print("Set preference %s to %s" % (key, repr(value)))
	_log("Set preference %s to %s" % (key, repr(value)))