208  Metadata (Gunnar's proposal)

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/209

In this pull request I've applied all the suggestions and fixes suggested in the previous one.

In this pull request I've tried to create a fluent API removing generators and proxy.
I tend to prefer this solution because  it seems easier to understand and to debug.

Some changes to the gridialect ended up in this patch...in the end I've put them in a separate commit.