Use Jekyll.logger instead of Jekyll::Stevenson to log things

Gives us more control over the log level, and we can handle new input via buffers (for example) so we know when to add newlines, etc.

I wasn't feeling too happy with Ruby's built-in Logger class (due to the formatting we have in place and the difficulty with overriding its methods to color the output) so I did this instead.

/cc @mattr-