Feature/memberships

# What

Add membership functionality


Memberships
=====

Memberships is the redbooth relation between organization and users containing the role information

```
|-------|         |------------|         |--------------|
| User  |   ==>   | Membership |   ==>   | Organization |
|-------|         |------------|         |--------------|
                      \
                    {role}
```

Lists Memberships in your visibility scope

```Ruby
  membership_collection = client.membership(:index)
  memberships = membership_collection.all
```

You can also filter by multiple params (see docs [here](https://redbooth.com/api/api-docs/#page:memberships,header:memberships-memberships-list) )

```Ruby
  filtered_memberships = client.membership(:index, order: 'id-DESC',
                                                   per_page: 50)
```

Fetch a especific membership

```Ruby
  memberships = client.membership(:show, id: 123)
```

Create a membership

```Ruby
  membership = client.membership(:create, organization_id: 123, user_id: 123, role: 'participant')
```

Update a especific membership

```Ruby
  membership = client.membership(:update, id: 123, role: 'admin')
```

Delete a especific membership

```Ruby
  client.membership(:delete, id: 123)
```