Adding method pushBack for Matrices

Adding the method pushBack that can be applied to a Matrix.  It is useful for some ML library methods.  The method wraps the push_back method in OpenCV and appends the bottom rows of the input matrix with the specified matrix.  Rows do not need to match, but columns and type must match.

Usage:
inputMatrix.pushBack(appendMatrix);

Example:
var a = new cv.Matrix(150, 64);
var b = new cv.Matrix(101, 64);
console.log("a: " + a.height() + ", " + a.width());  // a: 150, 64
console.log("b: " + b.height() + ", " + b.width());  // b: 101, 64

a.pushBack(b);
console.log("a: " + a.height() + ", " + a.width());  // a: 251, 64