[Tech Debt]: started the CLI work for PR#49

This pull requests includes the very basic work to start end-to-end testing the arithmetic CLI integration

https://github.com/jspm/jspm-cli/issues/49
Core functions should always take sensible function parameters.

I'd suggest a function signature like -  

```javascript
  core.bundle(moduleNameArithmetic, outputFile);

//eg core.bundle('a + b - c', 'build.js');
//     core.bundle('a + b'); // defaults to build.js
//     core.bundle('a'); // defaults to build.js
// the above would keep consistency with the current API, making this a non-breaking change which is best
```