Remove debug logging

It was making the tests noisy. Later I probably will enable a verbose
mode, but for now it is just gone.
Normalise path separators

filepath.ToSlash switches all path separators to "/". This way we can match
files across operating systems!
Fix getPreviousDayWordCount

It was not actually using a timestamp that made any sense (way into the future),
effectively just grabbing the previous count.
Add test suite.

Long over due. Actually how I discovered the missing .Unix() and .ToSlash()
from the previous fixes :)