Conserve memory by using __slots__ instead of dicts

[sagivmalihi]

Currently each document stores it's dict and also a dict stored in _data that holds a key-value mapping for all fields.

I suggest to replace dict with slots specifying only the necessary attributes, and replacing _data with a "strict dictionary" (that holds the keys once per class, and only values on each instance).

This will also have the added benefit of turning Documents (that are not DynamicDocuments) to strict classes (not allowing undeclared fields, thus preventing mistakes).

[sagivmalihi]

Please take a look at the referenced pull request (#625) - it implements this feature with an absolutely backwards-compatible way. In my use case it gave a 10X overall memory reduction.