Add support for model validation

[sugarb]

perhaps something like is_(not_)valid_with. To replace the following:

specify_with('foo', 'bar', 'baz' )      { is_expected.to be_valid }
specify_with('foo', 'bar', :invalid)    { is_expected.not_to be_valid }

Replacement might look like:

is_valid_with     'foo', 'bar', 'baz'
is_not_valid_with 'foo', 'bar', :invalid