Use flat_map instead of map followed by flatten

Not only is this more idiomatic, [it's also better performance wise!][1]

[1]: http://gistflow.com/posts/578-use-flat_map-instead-of-map-flatten
Update contributing guidelines

Update all markdown links and reorder some guidelines.
Fix #18

Avoid creating strings with weird characters by using a different git
log format.
Remove unnecessary method