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/
This can be simplified a bit by getting both the key and the value:

```go
for k, v := range submission.Problem {
  // ...
}
```

For clarity, perhaps we can call the local variables `name` and `contents` or something like that. Especially now that the map will be called `submission.ProblemFiles`, `name` and `contents` should be clear enough (`filename` and `fileContents` would probably be too much of an echo).