Add `to_s` methods

When there are no to_s methods tests fail with:

expected: "x^2 + 2"
            got: #<Polynomial:0x007fe365284f10 @coefficients="x^2 + 2">

In the README.md there is also the to_s method:

    Polynomial.new([-3, -4, 1, 0, 6]).to_s #=> '- 3x^4 - 4x^3 + x^2 + 6'
    Polynomial.new([1, 0, 2]).to_s         #=> 'x^2 + 2'