Expose and allow to pass in sinon instance

[stoically]

Allows to work around #67 if one uses the API directly to create a sinon-chrome instance. Here's an example how I'm doing it.

Might be worth thinking about exposing that possibility to the default way of getting an instance too.

[acvetkov]

@stoically Great work! I've pulished new version sinon-chrome@2.3.1

[stoically]

Thanks for the fast merge and release!