Added a constant file
Moved PI from Quaternion.cpp to Constant.h
Also removed PI_2
Added toAngleAxis function
Doc fix
Added fromMatrix, updated toEuler
Added toMatrix
Matrix doc (very light)
Removed alone 0 digit from float
Renamed Quaternion vector component
'v' was not very clear, so using 'i' (as 'imaginary') component.
(Still, Quaternion shouldn't be modified directly)
Added fromMatrix