Removes code that does not make any more sense since the REST API is in place

This code parsed the html response from PyPLN and extracted data from there.
Since the response is now a well formated json, this code is not needed
anymore.
Adapts the corpus creation method to use the new REST API
Returns the response content (as a dictionary) from PyPLN.add_corpus
Adapts corpora list method to the new API
Clean up a test
Renames a test class
Starts refactoring Corpus objects
Remove mocked return value that shouldn't exist in a test
Allows user to instantiate a corpus from a url
Allows user to add documents to a corpus
Removes "documents" method from Corpus

We don't need since it is set in the constructor (documents is a list in the
json object returned by the API).
Sets Corpus.auth when instatiating from a url
Documents the possible values for the 'document' arg in Corpus.add_document

`requests' takes either a file-like object or a string, and it sounds like a
good idea for us to also support both, but this may lead to some confusion.
This docstring is suppose to make things a bit clearer.
Allows user to add more than one document with one api call
Adds basic document structure
Allows the user to list a document's properties
Improves consistency in the example used for the tests
Allows user to get specific properties from documents
Improves consistency in the example used for the tests
Compares more than the urls when comparing two documents