Start refactoring JulianDate

Turn all prototype functions (except for clone/equals/eqaulsEpsilon/compareTo) into static functions.
Merge remote-tracking branch 'origin/master' into refactor-julianDate
Ongoing JulianDate refactoring

Default constructing a JulianDate no longer creates an instance set to the current time; I added JulianDate.now() for this purpose.
Ongoing JulianDate refactoring.

1. Clean up documentation.
2. Make all relevant function take result parameters.
Optionally construct Viewer with a specified DataSourceCollection.
Update jsdoc to 3.3.0-alpha7

This resolves issues with `npm install` on some Windows machines.
Merge remote-tracking branch 'origin/master' into refactor-julianDate

Conflicts:
	Source/DynamicScene/SampledProperty.js
Refactor LeapSecond to be a simply data object.

1. Removes the need for circular referneces between `LeapSecond` and `JulianDate`.
2. Moves array of leap seconds from `LeapSecond.leapSeconds` to `JulianDate.leapSeconds`
Changed Scene.globe to scene.globe in b28 changelog
JulianDate doc cleanup.
Swap order of parameters in JulianDate.getSecondsDifference and JulianDate.getDaysDifference
Update changes
Merge remote-tracking branch 'origin/master' into constructWithDataSources

Conflicts:
	CHANGES.md
Merge pull request #1815 from AnalyticalGraphicsInc/constructWithDataSources

Optionally construct Viewer with a specified DataSourceCollection
Merge pull request #1820 from cmorse/master

Changed Scene.globe to scene.globe in b28 changelog
Minor doc tweak.
Tweak the font size of code elements in doc.  Fix #1813.
Merge pull request #1822 from AnalyticalGraphicsInc/issue1813

Tweak the font size of code elements in doc.  Fix #1813.
Fix typos
cleanup