Changes:
1. Javascript decoder can now be built separately for meshes and point
clouds. Together with other optimizations this results in about 33%
smaller Javascript for mesh decoder. (pre-built javascript for meshes
and point clouds is included).
2. Fixing bug in our symbol_encoder that resulted in failed encoding for
certain cases when there were around 1<<18 symbols.
3. Changing default setting of our draco_encoder app to -cl 7
1. Javascript decoder is now exported as a module using DracoModule()
function that needs to be instantiated on the client.
2. Updated Javascript example applications and README.md
3. Added normalization function to VectorD
4. Added support for converting a single signed value to symbol for
entropy coding and vice versa
5. Minor code cleaning
