allow rails users to have non-active_record forms
Merge branch 'safer-rails' of https://github.com/saturnflyer/reform

Conflicts:
	CHANGES.md
explicitly require Reform::Form::ActiveModel so that it doesn't include ::ActiveModel by mistake. closes #18.
ditching 1.8.7 CI.
remove automatic delegation for Form->Fields in favour of explicit forwarding.
change Form.new to only accept one attribute: the composition. we no longer provide support for a separate Representer instance as this showed to be clumsy. override Form::representer_class if you wanna inject your own.
Form::DSL now only creates the composition for us.
Form now provides ::property and ::properties and delegates them to the representer_class.
by making DSL optional, this push lets you define the form properties within the form but allows passing in a single composition object. you don't have to use :on anymore if you composition provides the appropriate API.
adding Form#to_hash and #from_hash, which used to be called #update_with to make forms (almost) nestable.
add tests for nested forms.
add validation for nested forms.
make form.errors.messages work in >= 3.0.
rename composition to model internally. move Composition to separate file. don't pass mapper to #setup_fields anymore.
move creation of property accessors to Form::property.
Form#to_nested_hash now returns symbolized keys.
no longer delegate property accessors to the model but to @fields. this was a bug, when calling #validate the model should remain untouched, but we already set the attributes.
simplify coercion test.
make nesting almost dynamic ;)
make #setup_fields generic when creating nested forms.
adding Form::form to declare nested forms.
removing Form#name as we have it in the Definition instance.
add test for renaming forms, currently not working.
remove redundant #nested_forms method.