allow_value: pre-set attributes before validation

While attempting to add support for `ignoring_interference_by_writer` to
the confirmation matcher, I was noticing that there are two attributes
we are concerned with: the attribute under test, and the confirmation
attribute -- for instance, `password` and `password_confirmation`. The
way that the matcher works, `password_confirmation` is set first on the
record before `password` is set, and then the whole record is validated.
This is fine, but I also noticed that `allow_value` has a specific way
of setting attributes -- not only does it check whether the attribute
being set exists and fail properly if it is does not, but it also
raises a CouldNotSetAttribute error if the attribute changes incoming
values. This logic needs to be performed on both `password_confirmation`
as well as `password`.

With that in mind, `allow_value` now supports a `values_to_preset=`
writer method which allows one to assign additional attributes unrelated
to the one being tested prior to validation. This will be used by the
confirmation matcher in a future commit.

This means that `allow_value` now operates in two steps:

1. Set attributes unrelated to the test, raising an error if any of the
   attributes do not exist on the model.
2. Set the attribute under test to one or more values, raising an error
   if the attribute does not exist, then running validations on the
   record, failing with an appropriate error message if the validations
   fail.

Note that the second step is similar to the first, although there are
more things involved. To that end, `allow_value` has been completely
refactored so that the logic for setting and validating attributes
happens in other places. Specifically, the core logic to set an
attribute (and capture the results) is located in a new AttributeSetter
class.

Also, the CouldNotSetAttributeError class has been moved to a namespace
and renamed to AttributeChangedValueError.

Finally, this commit fixes DisallowValueMatcher so that it is the true
opposite of AllowValueMatcher: DVM#matches? calls AVM#does_not_match?
and DVM#does_not_match? calls AVM#matches?.

lib/shoulda/matchers/active_model.rb

@@ -3,6 +3,14 @@
 require 'shoulda/matchers/active_model/validation_matcher/build_description'
 require 'shoulda/matchers/active_model/validator'
 require 'shoulda/matchers/active_model/allow_value_matcher'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_changed_value_error'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_does_not_exist_error'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_setter'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_setter_and_validator'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_setters'
+require 'shoulda/matchers/active_model/allow_value_matcher/attribute_setters_and_validators'
+require 'shoulda/matchers/active_model/allow_value_matcher/successful_check'
+require 'shoulda/matchers/active_model/allow_value_matcher/successful_setting'
 require 'shoulda/matchers/active_model/disallow_value_matcher'
 require 'shoulda/matchers/active_model/validate_length_of_matcher'
 require 'shoulda/matchers/active_model/validate_inclusion_of_matcher'