code: function isIdentity() {
           var a = this.val;
           return a[0] === 1 && a[1] === 0 && a[2] === 0 && a[3] === 0 && a[4] === 1 && a[5] === 0 && a[6] === 0 && a[7] === 0 && a[8] === 1;
         }
docs: returns true if the matrix is an identity matrix.
  @name isIdentity
  @memberOf me.Matrix2d
  @function
  @return {Boolean}