Replace all calls to puts with a logger

This means that we'll easily be able to write to a log file, if we
choose to, and also means that we can easily silence the output too.
Add a 'quiet' option to disable output entirely
Add an -o parameter to write log output to a file

Passing `-o foo.txt` will cause all of Varnisher's output to be written
to a file called foo.txt in the working directory.

Otherwise, output will be written to STDOUT as usual.
Merge pull request #8 from robmiller/logger

Refactor logging output