Create a postgresql datastorage to verify datastore API design

[hpoul]

[fgutmann]

Implemented this and merged into master. I encountered some problems and suggest a refactoring of the storage API. Or maybe we'll just need to add another higher-level layer above the the current storage API that can be directly implemented.

I created a wiki entry to track the current state:

https://github.com/dc2f/dstore-java/wiki/PgStorageBackend