Default values

https://github.com/paulelliott/fabrication/issues/158

I implemented a failing test for the feature and I tried to make it pass, but I have an error. I am not sure if I am implementing it where it should. Any feedback would be appreciated.

The implementation:
https://github.com/Nerian/fabrication/pull/new/default_values#L4L62

The test:
https://github.com/paulelliott/fabrication/pull/159/files#L5R222

The error:

```ruby
 1) Fabrication::Schematic::Definition#transient accept default value
     Failure/Error: transient :one, :two => 'with a default value'
     NameError:
       uninitialized constant Fabrication::Schematic::Evaluator::Hash
```