Eigen AxisAngle patch

I fixed Rot3::LogMap for quaternions in a0be48e. However, that might indicate a problem with the Eigen::AxisAngle(Quaternion) constructor. Specifically, the case for q.w()==1 is actually a 180 degree rotation, and I'm not sure it's handled correctly in Eigen. This is not strictly a GTSAM problem, but perhaps good practice to diagnose and contribute a patch to Eigen, as @richardroberts has already done. Also, good preparation for issue #138, added just now.

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