Optional Hasher for int64 type

Hi, thanks for awesome work! Bigcache working beautiful in production with almost 1 million rpm :)

I'm storing items in cache based in their id (int64 type), but Hasher interface supports only `func Sum64(s string) uint64` method, and because of what I implemented it as follows:
```go
// Sum64 decodes integer from byte representation of string
func (d DummyHash) Sum64(s string) uint64 {
b := []byte(s)
x, _ := binary.Varint(b)
return uint64(x)
}
```
Overhead isn't noticable, but looks hacky. What's your thoughts on this? Did I miss something? Could we extend interface to support int64? I would love to do pr myself, with your help and advice.