IPLookup: First rough version based on RobTex's API

**What does your instant answer do?**
 Reverse DNS from IP to domain name

**What problem does your instant answer solve (Why is it better than organic links)?**


**What is the data source for your instant answer? (Provide a link if possible)**
RobTex.com API 

**Why did you choose this data source?**
Available!

**Are there any other alternative (better) data sources?**
Not that we know of

**What are some example queries that trigger this instant answer?**
reverse dns 123.45.67.89

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
sysadmin/network geeks

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**


**Which existing instant answers will this one supersede/overlap with?**
None that I know of

**Are you having any problems? Do you need our help with anything?**

![image](https://cloud.githubusercontent.com/assets/978048/3424536/8cb81c9e-ffd4-11e3-9026-bf36142cb546.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://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[x] Tested cross-browser compatability

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [] Google Chrome   
        [] Firefox         
        [] Opera           
        [] IE 10           

    - Windows 7
        [x] Google Chrome   
        [x] Firefox         
        [x] Opera           
        [] IE 8            
        [] IE 9            
        [] IE 10           
        [x] IE 11           

    - Windows XP
        [] IE 7            
        [] 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
```

cc @zekiel 
I would check if `api_result` exists, too, since `ddg_spice_iplookup` can be called without any parameters. https://github.com/duckduckgo/duckduckgo/blob/master/lib/DDG/Rewrite.pm#L173