provide commonly used vars to function-based style properties by placing them in scope
this is done by wrapping the function in another function that establishes the scope
need to fully re-parse styles in worker before rebuilding
add support for custom properties per style-rule, that can be exposed to function-based filters, colors, etc.
rename Scene.rebuildTiles() to just Scene.rebuild()
rename Scene.reloadScene() to Scene.reload()
Merge branch 'master' into stylefuncs
avoid initial failure to initialize data source in web worker
Merge pull request #47 from tangrams/stylefuncs

Add custom properties to function-based styles, and simplify method signatures