Avoid using a global BlankSlate class

This caused some trouble when migrating a Rails 2 application to Ruby 1.9:

```ruby
1.9.3p385 :008 > nil.andand.to_s
 => "#<AndAnd::MockReturningMe:0x0000001ed54ad0>" 
```

While it is fine on a plain Ruby 1.9.3 IRB (`nil.andand.to_s` is `nil`, as it should be), it fails when there is a (global) `BlankSlate` class -- which Rails 2's `ActiveSupport` defines, for example.
I'm not sure if this happens only on Rails 2 + Ruby 1.9, but I'm pretty sure we never want to use a global `BlankSlate` class, so we're no longer doing that. :)

The pull request also includes some changes to make specs run on Ruby 1.9, and makes a spec more robust which expects an error to happen.

Specs are passing for Ruby 1.8.7, REE and Ruby 1.9.3: https://travis-ci.org/makandra/andand