Added Tag support and changed the Numeric <-> String Bijections to Numeric <-> String @@ Rep[Numeric]

not finished, still needs the String -> Option[String @@ Rep[X]] machinery and tests, but basic support is there and working:

    scala> import com.twitter.bijection._
    import com.twitter.bijection._

    scala> Bijection[Int, String](100)
    <console>:11: error: diverging implicit expansion for type com.twitter.bijection.Bijection[Int,String]
    starting with method inverseOf in trait LowPriorityBijections
                  Bijection[Int, String](100)
                                    ^

    scala> Bijection[Int, String @@ Rep[Int]](100)
    res1: com.twitter.bijection.package.@@[String,com.twitter.bijection.Rep[Int]] = 100