Goodie : Weekdays between two dates

# Goodie Pull Request Template

**What does your instant answer do?**
It calculates the number of weekdays starting from one date to another, both inclusive.

**What problem does your instant answer solve (Why is it better than organic links)?**
There are no organic links available which can do this calculation, or maybe there are but user will have to do some more work.

**What is the data source for your instant answer? (Provide a link if possible)**
No data source required.

**Why did you choose this data source?**
NA

**Are there any other alternative (better) data sources?**
NA

**What are some example queries that trigger this instant answer?**
weekdays from March 4 2014 to March 24 2014

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Everyone, not specific to any community.

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
Not sure. I saw this on github issues and gave it a try.

**Which existing instant answers will this one supercede/overlap with?**
Not that I know of.

**Are you having any problems? Do you need our help with anything?**
Code review will be helpful.

## 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 compatibility
(NA. No HTML is returned.)
    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
```