Allow autoRemove from api.subscribe based on callback return values

[jacogr]

Closes https://github.com/ethcore/parity/issues/3586

[ngotchac]

This would mean that something like this:

contract.Event.subscribe({ toBlock: 'pending' }, (logs) => {
  if (logs.type === 'mined') {
    doStuff(logs);
  }
}, true);

would unsubscribe on first callback call, right?

Don't we want to test if the callback returns a boolean (true IMO) the unsubscribe?
Just seems weird to have to return something if our condition haven't met.

NB : an example with the PR would be nice I guess, so we can really appreciate how it is used.

[jacogr]

As discussed - changed check to true and explicitly check for boolean return.

[coveralls]

Changes Unknown when pulling 290a4a4 on jg-subscription-check into ** on master**.

[coveralls]

Changes Unknown when pulling 290a4a4 on jg-subscription-check into ** on master**.