Added instances, functions and syntax for Seq.

As [requested](https://gist.github.com/debasishg/5163116):

```scala
scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> val m: List[List[String]] = List(List("a", "b"), List("c", "d"))
m: List[List[String]] = List(List(a, b), List(c, d))

scala> m.sequence
res0: List[List[String]] = List(List(a, c), List(a, d), List(b, c), List(b, d))

scala> val l: Seq[List[String]] = Seq(List("a", "b"), List("c", "d"))
l: Seq[List[String]] = List(List(a, b), List(c, d))

scala> l.sequence
res1: List[Seq[String]] = List(List(a, c), List(a, d), List(b, c), List(b, d))
```