First pass at a better default configuration for Google AdSense.

These changes would break ads for anyone already using the Google AdSense provider, but I don't believe the provider was fully featured enough to actually be useful. I'll need to double-check the VIP repo
If the provider isn't set by the filter, allow the user to choose a provider from the admin interface.
Ensure our default provider actually exists. Otherwise, the sky will fall on our heads.
Merge pull request #56 from Automattic/google-adsense

Better default configuration for Google AdSense + a provider picker for the admin