Topic/370 shared field in trait

I've been playing with Spock and traits mix and I spotted an issue with `@Shared` field declared in a trait as reported in [issue 370](https://code.google.com/p/spock/issues/detail?id=370).

After some debugging AST transformations using `EmbeddedSpecification` it seems Spock doesn't see fields from trait and cannot do rename/initialization transformation for shared fields. I tried to modify the behavior, but traits are in general visible as interfaces and abstract methods (with setter/getter for fields). It is possible to get the field from a field and its annotation(s) with something like that:
```
Traits.findHelpers(clazz.interfaces[0]).fieldHelper.getFields().get(0).getAnnotations()
```
but "usual" modifications for shared fields (rename/change initialization place) would be risky to perform in a trait. I hope you will have some better idea what to do with that issue.

Btw, as Idea 14 doesn't allow to create (and use) classes/traits in a package named "trait" I renamed it to "traits".