Added 'sides', 'countBy', and 'roll' tests -- created Dice() constructor

#Changes
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## Main.js
- Created a **Dice()** constructor.
	- Contains **sides** property.
	- Contains **countBy** property.
	- Contains **roll** property. Function should properly do a random roll taking into account number of sides and whatever number we're counting by.

## Test.js
- Verifies that sides of die passed as first parameter to Dice().
- Verifies that sides are restricted to be between 6 and 20.
- Verifies that number to count by is passed as the second parameter to Dice().
- Verifies that the number to count by is no greater than 100.
- Verifies that a number rolled on a die is between the lowest and highest possible rolls.

## Index.html
- Added **main.js** file to scripts.
- Added dropdown for selecting number of sides for die - *just for testing, will modify later*