Using the Timer Counter Block of the Arduino Due as a hardware counter

 Adding support to use the *Timer Counter Block* of the Arduino Due as a hardware counter with the TCLKx pins as clock input for the counters.

This pull request is not finished in the way that you should pull it right away, I just created it to move our discussion from the [specific commit](https://github.com/pklaus/DueTimer/commit/b6034b5#commitcomment-3383519) to this pull request because the commit&comment might get lost as I'll continuously rebase the branch on the current upstream master (thus auto-creating new commits). So @ivanseidel, please post your comment here again.