Add BigNumber support to expectEvent/inLogs

[nventuro]

• No support for BigNumbers
  The current equality test fails for BigNumber objects (which have an equal method). chai-bignumber supports these assertions with the chain number.should.be.bignumber.equal(otherNumber).

[nventuro]

Superceded by #1169 and #1189

[frangio]

Reopening because we have no issue to track supporting BigNumber equality in expectEvent.