Added Equals, GetHashCode, and Zero to Vector2

I've implemented Equals, GetHashCode, and Vector2<T>.zero.

Equals allows you to check if one Vector2's value is the same as another's. 
GetHashCode lets it be used in a dictionary effectively.
Zero returns a Vector2 with its X and Y set to 0.

The commit I made the desired changes to is 1540bb0566f3b2e613b8c31c392b368351ddf2f4

Feel free to ignore the changes to the project files. I use 2012 but did not change anything specific to that version.