Add Download feature

Added a submission "download" feature for #131. It saves problems and solutions using `/submissions/:key`. at exercism_home/solutions/:user/:language/:slug/:key/
The comment should start with `// Download ...`. See https://golang.org/doc/effective_go.html#commentary for the official style guide regarding documentation.

This will make the documentation useful on places like http://godoc.org/github.com/exercism/cli/cmd (or locally if you run `godoc -http=:6060` or whatever).

I'd recommend running `go vet ./...` and `golint *.go` on the project to catch common issues.