Measurement of the ratio of the production cross sections times branching fractions of $B_c^\pm \to J/\psi \pi^\pm$ and $B^\pm \to J/\psi K^\pm$ and $\mathcalB(B_c^\pm \to J/\psi \pi^\pm\pi^\pm\pi^\mp)/\mathcalB(B_c^\pm \to J/\psi \pi^\pm)$ in pp collisions at $\sqrts =$ 7 TeV