added Nightscape::Entity::Wallet class
added Nightscape::Entity::Wallet class

- new attr: balance
  - hash table: Rat indexed by CommodityCode
- new attr: subwallet
  - hash table: Nightscape::Entity::Wallet indexed by VarName
- new method: getbalance
  - get wallet balance recursively
- new method: setbalance
  - debit/credit wallet balance based on values extracted from
    Nightscape::Entry::Posting class

atweiden/nightscape