add HTML Entity Code answer module and test file

# 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?**
Returns the HTML entity reference (the code) for a query keyword (the character name).

**What problem does your instant answer solve (Why is it better than organic links)?**
Eliminates having to open a webpage to find the solution. Even if a webpage containing the required information is opened, locating the HTML entities you want is generally difficult.

**What is the data source for your instant answer? (Provide a link if possible)**
Source 1: http://dev.w3.org/html5/html-author/charref [W3.org]
Source 2: http://www.utexas.edu/learn/html/spchar.html [The University of Texas at Austin]

**Why did you choose this data source?**
W3 is the international standards organization for the web.

**Are there any other alternative (better) data sources?**
Other sources may be more navigable or easier to read, but W3 is more trustworthy to my knowledge.

**What are some example queries that trigger this instant answer?**
'html phone symbol', 'html code for em dash', 'html entity apostrophe', 'dollar sign html'

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Web developers, programmers

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
1. https://duck.co/ideas/idea/376/0click-plugin-for-html-entities
2. https://duck.co/ideas/idea/964/html-en-decoding-yes-that-s-different-from

**Which existing instant answers will this one supercede/overlap with?**
No known Goodies supercede or overlap.
-- May be of interest: Existing Goodie HTMLEntities.pm has the same triggers. But the expected inputs and functions are different and no known query triggers both at the same time. This module does 'name --> code'. HTMLEntities.pm does 'code --> name'. 
-- Another point of note: The query 'html delta' (asking for the code for the Greek letter) also triggers the HTML colors Goodie.

**Are you having any problems? Do you need our help with anything?**

**\*\*Note:** Please attach a screenshot for new instant answer pull requests, and for pull requests which modify the look/design of existing instant answers.
![screen shot 2014-06-10 at 2 21 24 am](https://cloud.githubusercontent.com/assets/6028224/3222585/2b6323e2-f018-11e3-867d-672ed80873a4.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)
[] 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
        [] Google Chrome   
        [] Firefox         
        [] Opera           
        [] IE 8            
        [] IE 9            
        [] IE 10           

    - Windows XP
        [] IE 7            
        [] IE 8            

    - Mac OSX
        [] Google Chrome   
        [X] Firefox  (by pasting answer HTML in Firebug)    
        [] Opera           
        [] Safari          

    - iOS (iPhone)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera           

    - iOS (iPad)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera            

    - Android
        [] Firefox         
        [] Native Browser  
        [] Google Chrome   
        [] Opera

    -  [X] Firefox on DuckDuckGo provided Debian environment
```

I worry that "html" is a bit too broad as a trigger word.  I can imagine that, say, `html reference` might a fairly common query which won't be handled here.