Writing area

https://trello.com/c/NElTBCxh/54-as-a-user-i-want-a-nice-place-to-write-things

![selection_034](https://cloud.githubusercontent.com/assets/8248100/5289749/cf72cf74-7b0e-11e4-91dd-73e344803606.png)

Filenames generally shouldn't have spaces, and should end in an extension. Something like `filename: "notecard-#{notecard.date}.txt"`.

Also, I want to shy away from from non restful actions like download. This should be a `show` action on a `DownloadsController` or similar. 

In the end, I think we should pull all of this (this controller) out of this PR. You titled the PR "Writing Area". It should not have anything to do with downloads. Again, this is solid work so save it on another branch to come back to, but for now I really want you to focus down on small discrete features that keep us moving forward.