add function offset()

[Dieterbe]

this one was trivial to write.
it's basically exactly the same as scale except we add instead of multiply

diff expr/func_scale.go expr/func_offset.go
10c10
< type FuncScale struct {
---
> type FuncOffset struct {
15,16c15,16
< func NewScale() GraphiteFunc {
< 	return &FuncScale{}
---
> func NewOffset() GraphiteFunc {
> 	return &FuncOffset{}
19c19
< func (s *FuncScale) Signature() ([]Arg, []Arg) {
---
> func (s *FuncOffset) Signature() ([]Arg, []Arg) {
28c28
< func (s *FuncScale) Context(context Context) Context {
---
> func (s *FuncOffset) Context(context Context) Context {
32c32
< func (s *FuncScale) Exec(cache map[Req][]models.Series) ([]models.Series, error) {
---
> func (s *FuncOffset) Exec(cache map[Req][]models.Series) ([]models.Series, error) {
40c40
< 			out = append(out, schema.Point{Val: v.Val * s.factor, Ts: v.Ts})
---
> 			out = append(out, schema.Point{Val: v.Val + s.factor, Ts: v.Ts})
42,43c42,43
< 		series[i].Target = fmt.Sprintf("scale(%s,%f)", serie.Target, s.factor)
< 		series[i].QueryPatt = fmt.Sprintf("scale(%s,%f)", serie.QueryPatt, s.factor)
---
> 		series[i].Target = fmt.Sprintf("offset(%s,%f)", serie.Target, s.factor)
> 		series[i].QueryPatt = fmt.Sprintf("offset(%s,%f)", serie.QueryPatt, s.factor)