Implement a /citations/doi enpoint

[RobSullivan]

Citation data from PubMed is also available as the is_ref_of and citation_count fields in a document.

It might be useful to know which articles citing a doi are free to access in PMC so a citations end point is needed.

There is a chance to discuss the best possible endpoint (making it RESTful, for example) but for now I think /citations/doi/:doi might work.

Please note this may not be the best way to implement it as I was learning as I was going along when developing this. Here's your chance to learn as you go too 😄

Gaps in my knowledge will be missing but I have tried to keep the steps as clear as possible. Keep an eye out for #2 commented out in the code and please don't hesitate to ask questions.

One possible way to approach this is to make this test pass.

1. Define a route path in app.js. As this app is built using Express, more on defining routes can be read here http://expressjs.com/guide/routing.html
2. Think of an appropriate name for the function to be called when this route path is requested.
3. In the routes directory create a new file and model the query to the database on this one making changes where necessary.
4. The response needs to have a header set to Access-Control-Allow-Origin: * (also see enable cors #11). I'll check on how to do this and edit this step soon. Or if you find the answer before I do please let me know 😃 update see http://expressjs.com/4x/api.html#res.set
5. Now I think that should be it. Start the app running and see what happens, or have a go at writing a test.