Add new parameter to the RingOptions to override CommandInfo first key

There is problem with `eval` and `evalsha` commands.
`COMMAND INFO eval` returns first key position equals `0`.
After that, redis ring chooses `eval` as a value for sharding.
They, if you try to delete created value, ring may choose another shard
and delete won't work.

Eval command should be parsed, to be sharded properly, according to
redis specs: http://redis.io/commands/command .

I've introduced a new flag in the `RingOptions`, which will enable new
behavior: `EnableKeyLocationParsing`.

If it is enabled, `cmdFirstKey` will try to get key position using
`cmd.getFirstKeyPos()`. This function is defined for `eval` and
`evalsha` commands.
If it has parameters, it will return `3`, otherwise it will return `0`.

ring_test.go

@@ -89,6 +89,23 @@ var _ = Describe("Redis Ring", func() {
 		Expect(ringShard2.Info().Val()).To(ContainSubstring("keys=100"))
 	})
 
+	It("supports eval key search", func() {
+		script := redis.NewScript(`
+			local r = redis.call('SET', KEYS[1], ARGV[1])
+			return r
+		`)
+
+		var key string
+		for i := 0; i < 100; i++ {
+			key = fmt.Sprintf("key{%d}", i)
+			err := script.Run(ring, []string{key}, "value").Err()
+			Expect(err).NotTo(HaveOccurred())
+		}
+
+		Expect(ringShard1.Info().Val()).To(ContainSubstring("keys=52"))
+		Expect(ringShard2.Info().Val()).To(ContainSubstring("keys=48"))
+	})
+
 	Describe("pipelining", func() {
 		It("returns an error when all shards are down", func() {
 			ring := redis.NewRing(&redis.RingOptions{})