Add get logs, plus various minor fixes

[mikemaccana]

Review and merge after #14 since this sits on top of that work. Once that's merged this will be much smaller.

• Add getLogs() which gets logs from a transaction
• Various minor fixes:
  • Need to set confirmation strategy explicitly
  • Remove an unused 'only' from tests
  • Remove not-not on null variable, since null is already falsy.