Ran Golint and fixed Bencode package import

Hello,

What I've done here is run [golint](https://github.com/golang/lint) which caught a lot of naming/documentation things which "violate" convention.

There was also a problem where `go build` would not run because the `bencode` the package can't be `go get`'d:

```
$ go build
tracker.go:4:2: cannot find package "bencode" in any of:
	/Users/nickp/code/golang-src/src/pkg/bencode (from $GOROOT)
	/Users/nickp/code/go/src/bencode (from $GOPATH)
```

I've renamed the import to `"github.com/mdlayher/goat/src/bencode"` but this package should either extracted out into it's own separate repo, or perhaps moved to `github.com/mdlayher/goat/bencode` because it isn't technically goat source, just a "helper package" that goat uses.

I've tried my best to correct fill in documentation, but feel free to change it.