- add a context_manager on Book to make the related session the default session
- added an hybrid_property field for each pair (field_denom, field_num) and refactored the whole creation of this decimal composity/hybrid type to dict_decimal
 - redefine __init__ of declarative_base to be able to add some additional processing (for instance convert "EUR" to session.query(Commodity).filter_by(mnemonic="EUR").one())
- when an active session is detected:
  - allow automatic lookup from string to Commodity in constructor
  - add automatically object to session
- implement lookup classmethod on each class that uses a "lookup_key" field to do the lookup (the field must be a String column)
- removed Budget from model_core
- add save/cancel methods on Book
- first draft of documentation of core objects model