add standalone make target, edit readme

A few changes to the build system and doc:

- updated Makefile (wasn't building as-is)
- added `standalone` target:
  - won't clobber `window.require` in modular projects
  - exposes `window.Retsly` if CommonJS/AMD not present
- made some edits to readme:
  - corrected build instructions (component doesn't install source files)
  - "Core SDK" example url and callback signature weren't quite right