Issue/250

This should fix issue #250.

I don't play piano, but I was willing to contribute to the update project and test github as collaborating tool. So please check any mistake. I've compared the old PDF with the new one and I see a lot of improvements, but I may miss some basic knowledge of piano music notation.
I couldn't find a manuscript to use as reference...

Please note that there are still some warnings about rests collisions which I was not able to find. Also, I'm not sure if using e.g. g/rest is the best way to avoid rests collisions.
There's also a warning when the manual beaming is used during a change of staff.