Add backwards-compatibility and deprecate stuff

[marten]

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.

[marten]

I've opened #34 for the removal of the built-in specific operation types (interactors/commands/queries) in favor of generated classes.

The other thing this branch does is deprecate Pavlov::Helpers but maybe this should only be done when the backend is completed.

I'll close this pull request now without merging.