Creating new Population spice

**What does your Instant Answer do?**
Returns the most recent population data for a specified country

**What problem does your Instant Answer solve (Why is it better than organic links)?**
Users asking for populations in queries can get their answer faster / easier.

**What is the data source for your Instant Answer? (Provide a link if possible)**
World Bank. http://data.worldbank.org/node/9

**Why did you choose this data source?**
Used in other spices such as Rainfall. Provides population data for previous years so the spice can be expanded later on.

**Are there any other alternative (better) data sources?**
I believe that World Bank is the most comprehensive source.

**What are some example queries that trigger this Instant Answer?**
'population of usa'
'population canada'
'population of rus'

**Which communities will this Instant Answer be especially useful for? (gamers, book lovers, etc)**
People interested in populations of other countries.

**Is this Instant Answer connected to a DuckDuckHack [Instant Answer idea](https://duck.co/ideas)?**
https://duck.co/ideas/idea/4643/country-facts-especially-population

**Which existing Instant Answers will this one supersede/overlap with?**
Zanran has a trigger word for population, but from my testing it does not trigger.

**Are you having any problems? Do you need our help with anything?**
A review of what can be improved would be awesome. I was not sure on the best subtitle for the spice.

**What are the terms of use for the API? Will DuckDuckGo need specific authorization (e.g. an API key)? Are there any costs associated with API usage?**

The full terms are located here: http://data.worldbank.org/summary-terms-of-use
The one that seems applicable is: "You must include attribution for the data you use in the manner indicated in the metadata included with the data." And we provide a link to World Bank,

There are no API Keys or costs associated.


**What does the Instant Answer look like? (Provide a screenshot for new or updated Instant Answers)**
![ddg-populationspice](https://cloud.githubusercontent.com/assets/3364763/5137873/2179115a-7111-11e4-82da-5629f83bfa93.png)


## Checklist
Please place an 'X' where appropriate.

```
[X] Added metadata and attribution information
[X] Wrote test file and added to t/ directory
[X] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
[X] Verified that instant answer adheres to code styleguide (https://duck.co/duckduckhack/code_styleguide)
[] Tested cross-browser compatibility

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [X] Google Chrome
        [X] Firefox
        [] Opera
        [] IE 10

    - Windows 7
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 8
        [] IE 9
        [] IE 10

    - Windows XP
        [] IE 7
        [X] IE 8 * 

    - Mac OSX
        [] Google Chrome
        [] Firefox
        [] Opera
        [] Safari

    - iOS (iPhone)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - iOS (iPad)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - Android
        [] Firefox
        [] Native Browser
        [] Google Chrome
        [] Opera
```

* I ran it through IE8 for XP and there is a slight issue with the search bar vertical spacing, but this is not limited to the population spice.
Screenshot attached

![iexp](https://cloud.githubusercontent.com/assets/3364763/5138185/a74e02c0-7118-11e4-8f55-894efd92fa33.png)

Thanks for the feedback. I've updated the pull request.