Initial commit

Meet `logrus`. Proposal for a standard Shopify Go logger. Features:

* Level logging
* Warning (doesn't quit), Fatal and Panic sends errors to Airbrake
* JSON logging, that you can pass all the fields you want
* Different logging in development (nicely formatted, hope to make it coloured too) and production

@Shopify/stack: @wvanbergen @burke @graemej @justinplouffe

Other Go users at Shopify:

@snormore @fw42 @camilo @fbogsany @aybabtme @mkobetic @benbjohnson

Discuss!

There's still a good bunch of things I want to do (e.g. make Airbrake optional, support logging to Kafka, etc.) but I feel this is the first iteration that is useful.