Avoid dot notation for CZML properties `number`, `boolean`, and `string`

While not officially JavaScript reservered words, some tools can't handle properties named boolean, number, or string.  Changing them from dot notation to string based lookups works around these issues.
Merge pull request #1130 from AnalyticalGraphicsInc/fix-reserved-words

Avoid dot notation for CZML properties `number`, `boolean`, and `string`