Use #public_send to disallow hitting private methods

Fixes #28.

If the subject looks like this:

``` ruby
class Person < Struct.new(:name)
  private
  def inner_name
    "Inner #{name}"
  end
end
```

then `its` shouldn't be able to hit the `inner_name` method. The following test:

``` ruby
describe Person do
  subject { Person.new('John') }
  describe '#inner_name' do
    its(:inner_name} { is_expected.to eq 'Inner John' }
  end
end
```

should raise a `NoMethodError`.