Fix for creating square matrices and added identity property to the Matrix function

[MarkRedeman]

The Matrix constructor used the (undefined) cols variable when creating the appropriate array instead of this.cols.

There was a isIdentity parameter in the Matrix constructor but it wasn't used. I added the Matrix.identity(size).

[rockbot]

Hi - thanks!! Please add some tests for your code - it'll not only make sure your code works the way you think it does, but it'll also help newcomers know how to use the functionality you've provided :-)

Holler if you have questions!