Tried to merge edits together....
Merge branch 'master' of https://github.com/mdaines/viz.js

Conflicts:
	Makefile
	viz.js
Added ASM_JS=0 option so that builds work with ALLOW_MEMORY_GROWTH
Added necessary changes to allow for layout engine choice.
Fixed issues where viz.c tried to clean up, after the layout engine has already done so.

This causes an error with _free attempting to free memory that has already been freed, and everything fails.