Fix quat::Quat::from_angle_{x|y|z}

These functions were broken due to mistakenly using the full angle theta
in one place instead of theta / 2. This resulted in non-unit
quaternions that definitely did not rotate things correctly.
Fix comments on Quat::from_angle_{x|y|z} functions
Merge pull request #33 from zhanrnl/fix-quat-from_angle

Fix quat::Quat::from_angle_{x|y|z}