Tweak configuration for clarity

As I was reading through the project, I found myself adding and tweaking comments in order to clarify my understanding of things. I completely understand if this isn't something you want to pull into the project, but I figured I'd offer the patch(es) just in case.

The most important thing is probably line 10 in `configuration.go`, Golang has [excellent guidelines](http://golang.org/doc/effective_go.html#commentary) about how they want exported types, members, functions, and methods to be documented in order to work well with all the default godoc tools.

This is how bugsnag-go currently shows up:
http://godoc.org/github.com/bugsnag/bugsnag-go

To work with this locally, run `godoc -http=":4040"` on the command line, and navigate to http://localhost:4040/pkg/github.com/bugsnag/bugsnag-go

The salient bits are:

* Start comments with the name of the type, function, member, etc that is being commented. This makes the project nicely grep-able.
* Package comments should start with `Package <package-name> ....`.
* Each comment should start with a sentence that summarizes the most important point, then can be followed by more content that goes into details. Some tools extract the first sentence to display overviews.
* Plain text, no formatting, everything is used verbatim. For code snippets, indent them. That will ensure that they get displayed in a fixed-width font, even where the rest of the comments are not.

The `doc.go` file is a great place for the package-wide comments. These get put into the "overview" section of godoc, and can contain usage examples, configuration stuff, and anything else that will help people understand the project and how to use it.

Here's how the `doc.go` for a tiny CLI project looks: https://github.com/exercism/configlet/blob/master/doc.go

And here's what it looks like in the godoc:
http://godoc.org/github.com/exercism/configlet/configlet