Make executePendingJobs public

[Matthiee]

Fixes #31

[ekibun]

Can you provide some example code for unit test?

Execute pending job cannot work for marco tasks such as setTimeout and resolving dart objects. I think only creating a new instance for each Zone(or using threadpool to reuse the instance) can meet your requirements.

[Matthiee]

Hi @ekibun currently I'm using the branch in this PR and all I can say is that the async/await code is now executed in the correct Zone if we are able to call executePendingJobs from inside that zone.

Creating a new instance for each zone is not an option for us at the moment.

I'll try to provide some sample code for unit tests soon.