Initial submit for Issue #93, Scale billboards based on distance.  Includes minor fix for setting compiledShaderRotationPick variable.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master'

Conflicts:
	CHANGES.md
	Source/Scene/BillboardCollection.js

Signed-off-by: Alex Wood <abwood@gmail.com>
Added new type, NearFarScalar to represent a scalar value bounding
values between near and far camera distances. Added new uniforms,
czm_sceneMode and czm_eyeAltitude2D.  Various cleanup and optimizations
based on feedback from initial pull request.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue93

Conflicts:
	CHANGES.md

Signed-off-by: Alex Wood <abwood@gmail.com>
General cleanup and minor refactoring.
Renamed NearFarScalar members for consistency, also removed code bloat.
Fixed documentation generation error with @private usage in DrawCommand.
Renamed czm_eyeAltitude2D to czm_eyeHeight2D and storing
ShaderProgram._eyeHeight2D as a Cartesian2 for normal and squared distance.
Defaulting scaleByDistance to be undefined.

Signed-off-by: Alex Wood <abwood@gmail.com>
Minor documentation tweak.

Signed-off-by: Alex Wood <abwood@gmail.com>
Assigning a default value to DynamicBillboards; defaults are a function of
the default object's height.  Tweaking interpolation function for more
appealing scale transitions.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue93

Conflicts:
	CHANGES.md
	Source/Renderer/DrawCommand.js

Signed-off-by: Alex Wood <abwood@gmail.com>
Bug fix; _eyeHeight2D was improperly set for 2D Scenes.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue93

Conflicts:
	Specs/MockProperty.js

Signed-off-by: Alex Wood <abwood@gmail.com>
Cleanup from code review.

Signed-off-by: Alex Wood <abwood@gmail.com>
Removed unnecessary disabling of label scaleByDistance.
Billboard scaleByDistance is undefined by default.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue93

Conflicts:
	CHANGES.md

Signed-off-by: Alex Wood <abwood@gmail.com>
Cleaned up Billboard scaleByDistance Sandcastle example.  General cleanup.

Signed-off-by: Alex Wood <abwood@gmail.com>
Additional cleanup.

Signed-off-by: Alex Wood <abwood@gmail.com>
Merge remote-tracking branch 'agi/master' into issue93
Merge pull request #1074 from abwood/issue93

Scale billboards based on distance (Issue #93)
Added Alex Wood to CONTRIBUTORS.md
Reorg CHANGES.md
Add front and back face passes for translucent, closed geometry.