Create Cell.cs, Update Board.cs

I added the Cell class, some private helper properties and methods to
the Board class, and rewrote CanMoveInDirection method.
Update Board

I implemented the remaining methods of the Board class, and added a few
methods/properties of my own.
Remove References & Packages

Removing the unnecessary packages.config files and xUnit references from
2048Implementation and 2048ConsoleApp
Small Adjustments

No major changes; just style choices.
Style changes

I changed some variable names and added in some comments.
Merge pull request #1 from bondeluke/master

Implemented Board class