allow pitch min to lay head parallel with body

[donrestarone]

to test:

install changes:

git clone -b donrestarone-support-wider-range-of-motion-head https://github.com/restarone/pidog.git --depth 1
cd pidog
sudo python3 setup.py install

open a python shell

sudo python3
>> from pidog import Pidog
>> my_dog = Pidog()
>> my_dog.head_move([[0, 0, -90]], immediately=True, speed=100)