Add ZSCORE command (jgaskins#24)

* Adding ZSCORE command support

* oops...

spec/redis_spec.cr

@@ -199,6 +199,11 @@ describe Redis::Client do
       redis.zcount(key, "0", "+inf").should eq(3)
       redis.zcount(key, "(1", "3").should eq(2)
     end
+
+    test "returns the score of a member in a sorted set at key" do
+      redis.zadd(key, "1", "one")
+      redis.zscore(key, "one").should eq("1")
+    end
   end
 
   describe "hash" do

src/commands/sorted_set.cr

@@ -58,4 +58,8 @@ module Redis::Commands::SortedSet
   def zcount(key : String, min : String, max : String)
     run({"zcount", key, min, max})
   end
+
+  def zscore(key : String, value : String)
+    run({"zscore", key, value})
+  end
 end