Add "collection" and "member" methods

Add Rails routing style convenience methods for defining endpoints, eg:

Collections:

```ruby
class Project < Rapidash::Base
  collection :archived
end

# creates the method below which performs a
# GET /projects/archived
client.projects.archived!
```
Members:

```ruby
class Project < Rapidash::Base
  member :status
end

# creates the method below which performs a
# GET /projects/123/status
client.projects(123).status!
```

- [ ] Write specs
- [ ] Documentation