Fix file opening on windows

Fixes #19.

I used the `letter_opener` gem [as a reference](https://github.com/ryanb/letter_opener/commit/3a2e31eefc6515fe95ab0f1bc4fcfc9468d7c514) for fixing this. Turbulence now works on my Windows machine :)

If you want, you can just cherry pick the first commit or I can remove that second commit. I'm not sure if it was really warranted.