Add option to list all documents and corpora

It was downloading only the first page from the HTTP server. Now it looks for
`next` parameter returned and if parameter `full=True`, downloads every page
returned.
`Corpus.add_document` return `Document` object

Fix #23

Also changed the test behaviour for `Corpus.add_documents` method: mocking
`requests.Session.post` instead of `pypln.api.Corpus.add_document`