Modified Client.gain_access to set Client.document and added unittest

The `gain_access` method of the pmp_client.Client object previously would
request the home-doc and then not save it as Client.document. However,
the expectation is that after 'gaining_access', a user should be able
to immediately begin querying. Modified method to set this attribute
as a NavigableDoc based on homedoc. Also, was able to pull options from
the document and to retrieve the href for credentials using the
NavigableDoc's methods, instead of `get_dict`, thereby eliminating an
import statement.

Added unittests for this behavior.

Also, bumped Changelog and version and edited TODO.
Added further error handling for missing values on retrieving AccessToken inside  method. It's an important first step, so we want to know right away if it has failed.