enhanced org-pomodoro

Sadly I forgot to commit. I do not have time for partial commits now.
Main enhancements:

* notify for notifications
* creation of more variables
* sound playback
* cleaned up code
* now countdown instead of elapsed time
* automatically start long break after 4 pomodoros
* cancel the clock when pomodoro is killed
fix typo
add brackets to mode line
reset pomodoro count after long break finished
add/cleanup some linebreaks
clean mode line when no pomodoro is running
adds hook variables and an `org-pomodoro-started-hook`
Switch to GPLv3 and add correct comments
Clean up code

* clean up docstrings
* add some structure comments
* restructure file
Correct commentary
Create customizables
Add structure comment
Use 'alert instead of 'notify
Call a processes instead of synchrounous playback
Fix argument error for org-pomodoro
Update README.md
Merge pull request #1 from leoc/master

I've already respondend you by e-mail. Great that this piece of code is having somebody to look after!