values centered at 0.5 since version 0.2.1

[speleo3]

In versions <=0.2 (up to commit 518bd89), the neutral position was sending 0.0 values for all axes.

# git checkout 0.2
# python spacenavigator.py
Devices found:
        SpaceNavigator
SpaceNavigator found
SpaceNavigator connected to 3Dconnexion Space Navigator version: 1056 [serial: ]
   x +0.00    y +0.00    z +0.00 roll +0.00 pitch +0.00  yaw +0.00    t -1.00
   x +0.00    y +0.09    z +0.00 roll +0.00 pitch +0.00  yaw +0.00    t +1.64
   x +0.03    y +0.11    z -0.01 roll +0.00 pitch +0.00  yaw +0.00    t +1.65

Since version 0.2.1 (commit 8dead9a), the neutral position sends 0.5 for all values.

# git checkout 0.2.1
# python spacenavigator.py
Devices found:
        SpaceNavigator
SpaceNavigator found
9
1
1
1
1
1
1
SpaceNavigator connected to 3Dconnexion Space Navigator version: 1056 [serial: ]
   x +0.00    y +0.00    z +0.00 roll +0.00 pitch +0.00  yaw +0.00    t -1.00
   x +0.53    y +0.48    z +0.50 roll +0.50 pitch +0.50  yaw +0.50    t +0.79
   x +0.52    y +0.47    z +0.48 roll +0.40 pitch +0.43  yaw +0.50    t +0.80

I would prefer if the API stays invariant. Also, 0.0 makes more sense in my opinion.