Add UnmarshalJSON to pgtype.Int2

fixes https://github.com/jackc/pgtype/issues/153

int2.go

@@ -3,6 +3,7 @@ package pgtype
 import (
 	"database/sql/driver"
 	"encoding/binary"
+	"encoding/json"
 	"fmt"
 	"math"
 	"strconv"
@@ -302,3 +303,19 @@ func (src Int2) MarshalJSON() ([]byte, error) {
 
 	return nil, errBadStatus
 }
+
+func (dst *Int2) UnmarshalJSON(b []byte) error {
+	var n *int16
+	err := json.Unmarshal(b, &n)
+	if err != nil {
+		return err
+	}
+
+	if n == nil {
+		*dst = Int2{Status: Null}
+	} else {
+		*dst = Int2{Int: *n, Status: Present}
+	}
+
+	return nil
+}