Rewrite Preferences to make them properly async

[Snuffleupagus]

I've recently looked at the way that I implemented Preferences, and concluded that there is definitely room for improvement.
Particularly, they are not properly async, which is likely to cause issues at some point. For example, I think that Preferences.get really should return a Promise instead.

/cc @brendandahl: Before I begin rewriting this, I'd appreciate some feedback regarding how you would like Preferences to work.

[brendandahl]

Having get and set return promises would be ideal.