Add DNA reverse complement goodie

**What does your instant answer do?**
It returns the DNA [reverse complement](http://en.wikipedia.org/wiki/Reverse_complement) of a DNA or RNA sequence, with support for ambiguous bases.

**What problem does your instant answer solve (Why is it better than organic links)?**
This is a straight text transformation problem, so it's unlikely the user would be looking for a web search result.

**What is the data source for your instant answer? (Provide a link if possible)**
N/A

**Why did you choose this data source?**
N/A

**Are there any other alternative (better) data sources?**
N/A

**What are some example queries that trigger this instant answer?**
reverse complement of AATCG
revcomp AAUCGUAACGGGCCA

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Molecular biologists and bioinformaticists. Getting the reverse complement of a sequence is an everyday problem in these jobs.

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
Yes, [this one](https://duck.co/ideas/idea/165/reverse-complement-of-rna-or-dna?follow=forum_comments). 

**Which existing instant answers will this one supercede/overlap with?**
The 'reverse complement [of]' trigger also triggers the 'reverse' goodie, so both goodies return an answer. I'm not sure if this is a bug or a feature, although it's hard to think of a situation where the user would type 'reverse complement [of]' followed by a valid DNA/RNA sequence and be looking for the whole tail string (including the word 'complement') to be reversed.

**Are you having any problems? Do you need our help with anything?**
Would it be fair game for me to edit the 'reverse' goodie to prevent the problem above?

**\*\*Note:** Please attach a screenshot for new instant answer pull requests, and for pull requests which modify the look/design of existing instant answers.

![image](https://cloud.githubusercontent.com/assets/726198/3169570/c0ae5038-eba0-11e3-9197-249bb4ebb4af.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
        [] 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
```
@mintsoft Thanks for the review!  While I agree this (as a straight-forward text transformation) should be cached, it's not entirely clear from recent conversations that `is_cached => 1` is the default for Goodies.

@wilkox I would recommend changing to `is_cached => 1` in both this package and the tests for now. We can clean it up later if/when the Goodie default is clear.