OpenGL documentation fathead module.

# Fathead Pull Request

**What does your instant answer do?**
This is a fathead module for parsing the OpenGL documentation.  It should provide a short summary of OpenGL functions including the function prototype.  

**What problem does your instant answer solve (Why is it better than organic links)?**
Provides quick reference to the function prototype and direct link to the documentation page.

**What is the data source for your instant answer? (Provide a link if possible)**
DocBook XML files available via Subversion: http://www.opengl.org/wiki/Getting_started/XML_Toolchain_and_Man_Pages

**Why did you choose this data source?**
Well structured XML data for each OpenGL function.  Easily parseable with a good XML parser.

**Are there any other alternative (better) data sources?**
The xhtml web pages themselves could be scraped, but these pages are auto generated from the XML files, so we might as well use original source.

**What are some example queries that trigger this instant answer?**
`opengl glGet`
`opengl glBindTexture`

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Graphics/game programmers working with OpenGL.

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
No.

**Which existing instant answers will this one supercede/overlap with?**
None of which I am aware.

**Are you having any problems? Do you need our help with anything?**
Need help testing.

## Checklist
Please place an 'X' where appropriate.

```
[x] Added metadata and attribution information
[] Wrote test file and added to t/ directory
[] Verified that instant answer adheres to design guidelines(https://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[] 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
        [] 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
```
@daw42 Is there a reason to use svn?  We don't have it installed by default so I'm wondering if wget might work.