Add function to compute eigenvalues and eigenvectors for symmetric 3 by 3 matrices.
Add test to see if eigenvalues and eigenvectors are correctly computed.
Add more tests.
Add doc.
Update CHANGES.md.
Updates based on review.
Merge pull request #1224 from AnalyticalGraphicsInc/eigenDecomposition

Eigen decomposition
Merge remote-tracking branch 'origin/master' into skybox