Docs: Remove use of setDefaultAnnotationNamespace(), add "use" on documents.

- setDefaultAnnotationNamespace() was removed in Doctrine 2.2.
- remove reference to it in the introduction document.
- add namespace-aliased annotations on document examples.
Also updated getting-started.rst.
Merge pull request #634 from FGM/remove-setDefaultAnnotationNamespace

Docs: Remove use of setDefaultAnnotationNamespace, add "use" on documents.