Add '*' and '+' aliases for Point#multiply_by_scalar and Point#add_to_point

These aliases make code that depends on ECDSA::Point much more concise and elegant. For example, rather than the following code:

```ruby
point = point.add_to_point(other_point.multiply_by_scalar(scalar))
```

you'll now be able to write:

```ruby
point += other_point * scalar
```