Nuget switcharoo

This PR makes it even more clear that the old-and-busted Akavache backend shouldn't be used in existing code, by moving it to a separate Akavache package, `Akavache.Deprecated`. 

This means, that if we didn't change things, the `Akavache` package itself wouldn't be usable by itself, so the New User Experience would suck. So, instead, we move everything that was in Akavache into a new package `Akavache.Core`, then make `Akavache` into a metapackage that also includes `Akavache.SQLite3`.

## TODO

- [ ] Figure out what to do for WP8 people, who don't even have SQLite3