Update Syntax module

Not necessary to add a module for `ActiveRecord::Relation`. The Syntax
module can be included into `ActiveRecord::Relation` to acheive the
previous same functionality. Remove `register_mapping` API in favor of
defining query classes exclicitly as a subclass of `Paraphrase::Query`.
Move models to spec_helper

Easier to manage.
Allow passing a model class or relation to `Query`

Defer to `Scope#chain` to resolve whether `relation` is an
`ActiveRecord::Base` subclass or an `ActiveRecord::Relation`.
Install gems concurrently
Inline conditional for key method definition
Rename `Query#relation` to `Query#result`

Update the README example for using `Query` instances to drive forms.
Switch to 1.9 hash syntax
Add `form_for` support
Remove delegation to `result`