Add backwards-compatibility and deprecate stuff

I'm bound to have forgotten some features, so let me know. I'm trying to make it so that you can simply `require 'pavlov/alpha_compatibility' and have all old code keep working.

Stuff that isn't backwards compatible:

  * Instantiating operations outside of helpers: `Queries::Foo.new(arg1, arg2)` breaks, because it's the `Pavlov.query` etc methods that transform old-style `*args` into new-style hash.