Add ChangeLog
Bump version to 0.1.2
Add cancellation API
Test the new cancellation API
Update header documentation
Update README
Minor update
Rescue should proxy the progress and pick-up where failed
Possibility to run blocks on specific queues
Add task-based promise creation
Refactoring, improvements and additions.

- Promises created by then: and rescue: fail in case the supplied block
  returns an NSError or the block raises an exception. That's now the
  behavior for all blocks that return a value.
- then: is now aware of number of parent promises and can thus
  calculate the progress. It assumes an equal distribution of work
  over all promises.
- Add missing unit tests.
merge recent master changes
Finalize new chain: implementation.

- Add new tests and fix incorrect onces.
- Update chain:initial: implementation and update corresponding docs.
- Update documentation for fulfilled:, failed: and progressed:.
- Remove obsolete code.