Improve performance of double creation.

A couple of optimisations that speed up double creation significantly. See individual commits for their contribution details.

Time to create 1000 doubles:

<img src="https://dl.dropboxusercontent.com/u/8686208/rspec-mocks-issue-193-improvement.jpg" />

I just cranked away with benchmark and `ruby-prof` for a while, using code like the following:

``` Ruby
n = 1000
Benchmark.bm do |bm|
  RSpec::Mocks.setup(self)

  (0..9).each do |m|
    attrs = m.times.inject({}) {|h, x|
      h["method_#{x}"] = x
      h
    }

    bm.report("double") do
      n.times do
        double(attrs)
      end
    end
  end
end
```

**Notes**

* `caller` location has changed for `expected_from` definition. Pretty sure this isn't a problem.
* "simple stub" is a new concept, I believe it justifies itself in the speed improvement.
* Addresses #193.

R @myronmarston