Break out Oids into a new subpackage

To clean up docs, and also because some people wanted to re-use them
separately.

Signed-off-by: Daniel Farina <daniel@fdr.io>
Add extensible "codec" system for managing custom/composite/array
pg_types oid.

* Ability to register Decoder or Encoder funcs globally for all
  connections or build a set (Codec) of funcs that can be
  referenced in the connection string to Open()
* go1.1 only (relies on fix to goissue3640)
* Existing (pre-registered) types cannot be overriden by design
  for consistancy - might change this though.

Example:

```
	// func that takes postgres bytes and turns into a []int
	func int4SliceDecoder(b []byte) (driver.Value, error) {
		ca := make([]int, 0)
		for _, s := range strings.Split(string(b), ",") {
			ca = append(ca, s)
		}
		return ca, nil
	}

	// func that takes []int and returns raw []byte for postgres
	func int4SliceEncoder(v interface{}) (driver.Value, error) {
		switch x := v.(type) {
		case []int:
			s := make([]string, len(x))
			for i, n := range x {
				s[i] = strconv.Itoa(n)
			}
			return []byte(fmt.Sprintf(`{%s}`, strings.Join(s, ","))), nil
		}
		panic("unexpected type")
	}

	// create and register a new set of codecs
	c := pq.NewCodec()
	c.RegisterDecoder(1007, int4SliceDecoder) // 1007 is type oid
	c.RegisterEncoder(1007, int4SliceEncoder)
	pq.RegisterCodec("mydec", c)

	// get a DB that will use the codec
	db := sql.Open("postgres", "codec=mydec")

	// use our new array type
	arg := make([]int,1)
	arg[0] = 1
	rs,_ := db.Query("SELECT ARRAY[10,20,30] WHERE ARRAY[1] = $1", arg)
	for rs.Next() {
		var res []int
		rs.Scan(&res)
		// res[0] = 10
		// res[1] = 20
		// res[2] = 30
	}

```