DOI urls

Please ignore the changes by numm/baker/flores.

Our branch builds on theirs because their branch seemed to fix a bug for us related to DOI in master.

This is how we've implemented it so far:

You know the DOI of your paper. You go to this URL:

![image](https://cloud.githubusercontent.com/assets/5796310/5136130/b631a346-70d7-11e4-994f-4e4b9583d55b.png)

(The DOI is `publisher/journal.2`)

The page this requests in order to get the paper's JSON:

![image](https://cloud.githubusercontent.com/assets/5796310/5136146/e1d31642-70d7-11e4-894b-131d77e86509.png)

Does this seem reasonable to you all so far?

The overall goal of this PR is to support `paper.id` lookups as well as `paper.doi`.

In order to support this we will need to add more routes throughout the app that understand requests like the one above (`?doi=...`).

So far we know of one endpoint we will need to update:

![image](https://cloud.githubusercontent.com/assets/5796310/5136170/358be930-70d8-11e4-8879-f7dc2256ab9c.png)

— AP DT
Can you get rid of `id?` and do `Doi.valid?` instead?