Feature/api create and delete relationship

@mtakac why is this `match` here? Can't you do parameter validation in the `params` block above? e.g.

```ruby
optional :to_id, desc: 'Id of related work package', type: String, regexp: /\d+/
# or better
optional :to_id, desc: 'Id of related work package', type: Integer

```

https://github.com/intridea/grape#parameter-validation-and-coercion