Add task manager

[Gjum]

Also fix some tests on the way.

This is the beginnings of #147.

[Gjum]

There was a discussion if this belongs into core. I think this is not required to run the client, but rather nice to use. So I put it into helpers.

[gamingrobot]

+1 for having it in core, timer is not required for the client but again is nice to use.

[nickelpro]

core isn't "required for client to run" nor is it "nice to have". core is where things related to Spock as an event framework go, task manager is clearly an event framework feature and belongs in core.

helpers is where we put things that implement minecraft behavior or features. Other client implementations may or may not want these minecraft features, a server or proxy will likely want none of them. That is why they are easily and categorically seperated from core

[Gjum]

OK, moving to core then.