First observation of the decay $\bar{B}$$0\atop{s}$ → D$±\atop{s}$ K∓ and measurement of the relative branching fraction B($\bar{B}$$0\atop{s}$→ D$±\atop{s}$ K∓)/B($\bar{B}$$0\atop{s}$→ D$+\atop{s}$ π-).