Basic support for text symbolizers

This adds basic label rendering for points and polygons to the
canvas renderer, and a text symbolizer to the style package.
Fixing merge errors from unmerging #404
Use 'text' instead of 'name' for the label text
Changing renderLabels_ method name to renderText_
Proper handling of defaults
Adding TODOs
Updating example to use new properties
Merge pull request #770 from ahocevar/text-symbolizer

Basic support for text symbolizers. r=@bartvde,@tschaub