(Temporarily) add Traceur runtime as testing dependency

Allows for a lot more ES6 syntax but does complicate the build process.
Should end up doing one of the following:

- Note in the readme that you must include `traceur-runtime.js` in order
  for the library to work
- (Optionally?) Distribute the runtime with the built version of the library
- Replace Traceur with esnext once it has module support. esnext does
  not require a runtime library (AFAIK).