Optionally create tasks for running subsets of all tests (small etc)

Adds `spoon.useTestSizes` property. When it is enabled, tasks for running subsets of all tests will get create - one for each test size annotation (`@SmallTest`, `@MediumTest`, `@LargeTest`)

Here I also moved info about test class/method settings lower in README as that info is less important than what's above.

closes #19, closes #20