Working get_todos

It seems like a few rust changes broke `get_todos`. I had to point to my own fork of `nickel.rs` while I'm waiting for nickel-org/rust-http#2 to be merged. I'm happy to wait until that is merged and pointing back to mainline `nickel.rs` before merging this.

I implemented `ToJson` on `Todo` instead of on `Vec<Todo>` primarily because we get `Vec<Todo>.to_json()` for free when doing so. Also, it seems like it might be better to build a layer on top of the core json serialization stuff to get our output a little more `JSON API`-ish. I'm not sure if this is the direction we should go, so input here is greatly appreciated.