Proposal: simpler "include_dirs" bundle code

The old command is really verbose. It's time to consolidate it into a bundled node script. Our users will thank us.

__Before:__

``` python
"include_dirs" : [
    "<!(node -p -e \"require('path').relative('.', require('path').dirname(require.resolve('nan')))\")"
]
```

__Now:__

``` python
"include_dirs" : [
    "<!(node -e \"require('nan')\")"
]
```

I also updated the README with the new syntax, and updated the tests to utilize the documented method so that we can ensure it never breaks in the future.

The old syntax still works for backwards compatibility purposes. Cheers!