Fixed Assets Improvement

account_asset module improvement:

Current state of branch:
- Asset categories now hierarchical
- Asset methods types (cost basis, tax) used for distinguish various types of depreciations and used for default settings.
- Defaults for pairs (category - method type): Accounts, calculation methods, values...
- Calculation methods:
---- Straight line - works
---- Declining Balance - works
---- Sum of Year Digits - works
---- Units of Production - works
---- Progressive (?) - works but is needless probably

- Time Method: Only whole intervals synchronised with calendar by now.
Calculation of depreciation from specific date is under consideration but looks too chalengous.

- Will make wizard for liquidation and revaluation. Automatic tax depreciation method too chalengous but possible manually.

- refund invoice of cost basis - works
- sale of asset - will work

Improving asset history.
No idea how to implement analytic posting.