Use child_process spawn instead of exec

Hey @notatestuser,
I tried to implement the usage of `child_process.spawn` method instead of `child_process.exec` to avoid errors
as reported in #23.

I'm opening this PR so we can discuss the changes and fix possible errors - unfortunately I'm a Windows developer, so I couldn't run successfully all tests (I'm hoping Travis will tell me what's wrong).

Thanks in advance!

Fixes #23