Release 0.3.0

Changelog:

• ledger file schema change for improved readability and maintainability
• refactored the core module logic into struct methods to reduce redundancy and improve logic organization
• add improved error handling and messaging
• substitute integration tests with unit tests
• use floating point types @dantuck
• use monee crate in register and balance modules @dantuck
• split transactions (which informed the schema change named above) @dantuck