Created new Goodie: PercentOf

# Goodie Pull Request Template

Using this template will help us better understand your Instant Answer and assist you when necessary.  Simply copy and paste the markdown below into the description of your GitHub pull request and complete as appropriate.

**What does your Instant Answer do?**

Permits to calculate numbers with percentuals

**What problem does your Instant Answer solve (Why is it better than organic links)?**

Calculating numbers with Percentuals

**What is the data source for your Instant Answer? (Provide a link if possible)**


**Why did you choose this data source?**


**Are there any other alternative (better) data sources?**


**What are some example queries that trigger this Instant Answer?**

4+50% 234*16%


**Which communities will this Instant Answer be especially useful for? (gamers, book lovers, etc)**


**Is this Instant Answer connected to a DuckDuckHack [Instant Answer idea](https://duck.co/ideas)?**


**Which existing Instant Answers will this one supercede/overlap with?**


**Are you having any problems? Do you need our help with anything?**


**Where did you hear about DuckDuckHack? (For first time contributors)**


**What does the Instant Answer look like? (Provide a screenshot for new or updated Instant Answers)**

![screen](https://cloud.githubusercontent.com/assets/8344987/5019276/696a9064-6abe-11e4-8c19-47cd7df14443.jpg)


## Checklist
Please place an 'X' where appropriate.

```
[] Added metadata and attribution information (https://duck.co/duckduckhack/metadata)
[X] Wrote test file and added to t/ directory (https://duck.co/duckduckhack/goodie_tests)
[] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
[] Verified that instant answer adheres to code styleguide (https://duck.co/duckduckhack/code_styleguide)
[X] Tested cross-browser compatibility

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 10

    - Windows 7
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 8
        [] IE 9
        [] IE 10

    - Windows XP
        [] IE 7
        [] IE 8

    - Mac OSX
        [X] Google Chrome
        [X] Firefox
        [] Opera
        [X] Safari

    - iOS (iPhone)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - iOS (iPad)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - Android
        [] Firefox
        [] Native Browser
        [] Google Chrome
        [] Opera
```
I'm not sure if anchoring the `%` to the end of the query is ideal. Otherwise we can't handle a query like "15% * 43"