Default orientation for DataSource models

Eventually we plan on providing easy ways to specify orientation for Entity instances; but a small usability improvement in the short term is to just have the model stand up right if no orientation is provided.
Fix normal upsampling.  Cannot linearly interpolate oct encoded normals in all cases, due to how the octrahedron is unwrapped into a unit square.
Moved octEncoding into Core.  Moved signNotZero and toSNorm into CesiumMath.  Unit tests included.
Minor cleanup.  Added new conversion function to Math.js, fromSNorm.
Minor cleanup.
Merge remote-tracking branch 'agi/master' into upsampleQuantizedMeshNormalsFix
Merge branch 'issue1940' into dataSource-model-defaults
Doc updates.
Update default orientation after #1963
Cleaned up doc for Oct.
Merge remote-tracking branch 'origin/master' into dataSource-model-defaults
Fixed typo in doc.
Add rudimentary Timeline documentnation for 1.0

While the Timeline will be completely re-written in the future, we should have at least minor doc for 1.0 (though most things have been marked private).

CC #754
Fix bad if statement.
Documented exceptions in Oct.
Mark Oct class as private.
Merge remote-tracking branch 'agi/master' into upsampleQuantizedMeshNormalsFix
Minor cleanup.
Merge pull request #1961 from abwood/upsampleQuantizedMeshNormalsFix

Fix oct-encoded normal upsampling.
Reorder 1.0 in CHANGES.md