Implemented dynamic authorization by authenticate tokens that can be changed

at every request according to the current user.
Fixed HOST of dynamic auth for the case that no static auth configured.
Added tests for dynamic auth.
Implemented dynamic instance_url for dynamic auth.

It is not mounted together with max_retries=MAX_RETRIES as is usual for static
configuration.
Preserved order in get_classes_texts to improve reusability.
Merge pull request #80 from hynekcer/dynamic-auth

Dynamic authorization by authenticate token