Refactoring for running the tasks in different contexts

[veewee]

This major refactoring has some BC breaks.
It is possible to specify which task can run in which context.
A console helper is created that is responsible for running the tasks in the console commands.
This way it is easy to reuse the taskrunner in multiple console commands: You will only need to pass a new context.

This PR will make it possible to develop #33.

@aderuwe, @igormukhingmailcom : Can you please review and tell me if you like it or not?

[igormukhingmailcom]

@veewee Looks very cool. I like :)

[igormukhingmailcom]

Can you please tag it as 2.0.0 (now or in future when it will be stable) as it has BC break as you said.

[veewee]

Hello @igormukhingmailcom,
Since the project is not stable enough, I will be tagging stuff as 0.(major structural changes).(patch or small feature).
The PR is tagged as 0.4.0.
There are still some things I want to change in the codebase before tagging a stable major release.