208 Metadata

Previous pull request: https://github.com/hibernate/hibernate-ogm/pull/196
JIRA: https://hibernate.atlassian.net/browse/OGM-208
Alternative pull request: https://github.com/hibernate/hibernate-ogm/pull/210

In this pull request I've applied all the suggestions and fixes arised from the previous one.
In this commit it is still using the generators and proxy approach

Some changes to the gridialect ended up in this patch...in the end I've put them in a separate commit.