AMD and ES6 interop support

Under modules=amd, mark modules with property '__esModule', and treat all other modules as default exports to allow mixing ES6 and AMD modules under both ES6 and AMD loaders.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/785

Closes #785.
Followup cleanup from last commit
Print a stack trace when the interpreter sees an exception.
Fixes #812
Set the process.exit code to 8 on error