LeanTask (mini task), new methods for tasks

[Laxilef]

Inspired by the work of @dbuezas (#20), I added LeanTask to the current version of the scheduler and added some features.
This helps to really save RAM. On working existing projects, even 6 tasks completely clogged the heap, after which the esp8266 crashed.
The PR includes updating the documentation. Added a table with LeanTask testing results to the README.
I also added new examples. You can review them before merging branches (if you do).

Briefly:

1. Added AbstractTask with new methods: isEnabled, enable, disable, setInterval, getInterval
2. Task and LeanTask inherit AbstractTask
3. Work with the cont library has been moved to Task
4. "tramponline" function moved: static void SchedullerClass::trampoline
5. ::loop() can now be used (although not recommended)
6. Little refactoring
7. Readme updated
8. Version updated to 0.3 :)

[nrwiersma]

Looks good so far. A couple comments.

[Laxilef]

Hello

Done.

[nrwiersma]

LGTM 🎉

[Laxilef]

When can we expect a release? :)

[nrwiersma]

I am away at the moment and will be back mid next week. Alternatively you can create a PR bumping the version number and I can do the rest on my phone when I get a chance this weekend. Up to you.