• Fix incomplete Action implementation for BalanceAssertion.

• Refactor to properly support user defined Actions. This included making Program and a number of other types generic over ActionTypeValueEnum and ActionTypeEnum implementations. These changes shouldn't affect anyone using the provided set of Actions.
• Introduced a new trait ActionTypeFor which has some functionality separated from Action. This is a breaking change for people implementing Action themselves.

• Fix building for default features accidentally including some items only needed for serde-support.

• Complete serde serialization for Program, and the various Action implementations. #2.
• Refactor Program to use an enum ActionTypeValue instead of dynamic trait dispatch over Action. Programs using custom actions need to create their own implementation of ActionTypeValueEnum to provide store their actions given to Program.

• Fix changelog for v0.6.1

• Bump rust_decimal dependency up to using generic version 1 to address #5.
• Update Account#new(), Transaction#new() and Transaction#new_simple() to use Into<String> to address #4.

Renamed argument in sum_account_states(), sum_currency to sum_commodity_type_id to better match the recent changes in commodity library.

• Updated commodity library dependency to v0.3.0, renamed some types
  which were changed for that version
• AccountingError::Currency renamed to AccountingError::Commodity