Make Entity.Meta.api_path simpler

Each entity's Meta.api_path is currently a tuple in this form:

    api_path = (
        '/api/v2/this_entity/',
        '/api/v2/this_entity/alternative_path',
    )

However, entities offer absolutely no support for intelligently choosing among
these URLs, which leads to confusing and ugly client code like this:

    return self.Meta.api_path[0]

This is a bad state of affairs. Convert each `api_path` to a simpler form:

    api_path = '/some/authoritative/path'

And trim down client code to deal with these changes.
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().path('this') => NoSuchPathError
    Organization(id=25).path() => 'example.com/api/v2/organizations/25'
    Organization(id=25).path('all') => 'example.com/api/v2/organizations'

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

Add tests for this new method, and fix the setUp/tearDown methods for some
existing tests.
Split lines as per PEP 8.
Merge pull request #1053 from Ichimonji10/entity-path

SomeEntity().path()