Added instance method rref for Reduced Row Echelon Form transformation

[dgendill]

I took the Reduce Row Echelon Form code from Roseta code and ported it to work with Sylvester.

Example test case:

test("rref - Reduced Row Echelon Form", function() { with(this) {
    var testMatrix = Matrix.create([[5,0,0,1],[0,5,0,2],[0,0,5,3]]);
    assert(testMatrix.rref().eql([
        [1,0,0,0.2],
        [0,1,0,0.4],
        [0,0,1,0.6]
    ]))
}})

I added the instance method, test case, and the method to the documentation. I wasn't able to rebuild the site files because I was getting a ruby error that I wasn't able to figure out. Maybe someone else can run the site build command.