feat: error handling + reorg

[driemworks]

Closes #8

• adds error handling for murmur-core/lib
• more robust unit tests + edge cases
• reorgs the crates to include the cli as a binary instead of a standalone crate
  • removes the cli directory completely and makes it usable as 'murmur' rather than 'murmur-cli
• also includes general improvements to the CLI, including configurable recipient of balance transfers

[juangirini]

LGTM