TODO added
core: Add stubs for most important class bindings

Some TODOs added.
accessor.hpp: Add macro for single entity getters

Macro creates a getter that returns an boost::optional instead of
an entity. Is used in order to get None in python instead of an
uninitialized entity.
entity.hpp: functions exposin EntityWithMetadata

- Define a function that can expose specialisation of
  EntityWithMetadata<T>
- Let block inherit from EntityWithMetadata<IBlock>
entity.hpp: exprter for EntityWithSources<T>

- Define exporter for entity with sources
  Just defines private functions that have to be
  used by some not yet implemented proxy

- Add macros to accessors.hpp
  It's not getting nicer with all the macros

- Let DataArray inherit from EntityWithSources<IDataArray>
Merge pull request #3 from stoewer/master

Bindings for EntityWithSources and EntityWithMetadata

Sweet!