[#76505670] Replace stdlib log package with logrus

Replace the `log` package from Go's standard library with [logrus](https://github.com/Sirupsen/logrus), primarily because it supports logging levels.

The default logging level outputs warnings and above. For more logging detail, use the `-verbose` flag. The `-quiet` flag outputs errors only, and the `-debug` flag will output debug messages (we currently don't have any).

Logrus also supports Airbrake/Errbit and JSON-formatted logs, which we can parse with [logstash](http://logstash.net/).

TTY output looks like:
![screen shot 2014-08-07 at 22 48 00](https://cloud.githubusercontent.com/assets/4348848/3849753/86c4d392-1e7c-11e4-8c48-d93447c0ef3b.png)

Note that I have only changed the main package to use these levels so that the other packages remain portable.

Also, move much as possible of the package initialisation to the `init()` function.

* * *

Also, prefer `log.Fatalln` to `log.Fatal` for consistency with our use of `log.Println`. Ensures that a new line is printed after the log message.