add bin files for easier execution and fix path problems

This allows you to install globally and run in any directory.

```bash
> intern-client config=tests/intern
```

or

```bash
> intern-runner config=tests/intern
```

This also allows you to install intern in your devDependencies and supply the npm test command since npm uses local bin files during scripts phases.

```js
{
  "scripts": {
    "test": "intern-client config=tests/intern"
  }
}
```