commonjs wrapper / get rid of globals

[bmeck]

probably just going to wrap in a different context for first iteration, i can take the issue that far at least. This will have primitives not matching outer context though (which is fine for first iteration).

cajaVM should not be a global, and should have a per instance shared import cache.
API could be something like:

  ses.createVM()

VMs need to provide:

  exports.eval(code) // no way to set the sandbox?
  exports.compileExpr(code, sandbox)
  exports.compileModule(code)(sandbox)
  exports.Function(args..., code)

Have prototype, reading up on caja stuff but docs are inline...