Add instance method Entity.path

By default, this method returns the path used to access all entities of its
kind, but one can also ask for an individual entity's path.

    >>> Organization().path()
    'example.com/api/v2/organizations'
    >>> Organization(id_=25).path()
    'example.com/api/v2/organizations'
    >>> Organization(id_=25).path('this)
    'example.com/api/v2/organizations/25'

If one asks for a path which cannot be constructed, an exception is raised.