Fix for Issue #2 the HandOffsetZ value is negative in Z axis

When comparing the nao forward kinematics results of this library and those from naoqi, I noticed an error in the Z axis value.
According to the aldebaran documentation HandOffsetZ is negative in the z axis. So I changed the code accordingly, and the bug was fixed.

After fixing the bug:

x = 218.7 y = 113 z = 87.69
x = 218.7 y = -113 z = 87.69

Naoqi output for the Left arm in the same configuration in meters:

x = 0.21852196753025055,
y = 0.11063870042562485,
z = 0.08774302154779434,