Added instance method rref for Reduced Row Echelon Form transformation

I took the [Reduce Row Echelon Form](http://rosettacode.org/wiki/Reduced_row_echelon_form#JavaScript) 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.