Tasks api

Implemented code for `entities.Tasks.poll`.
Implemented the code that allows you to poll the status for an aysnc
task (such as a repository synchronization for example). The method
'polls' for the status of the task and returns it once it is no longer
running (`'pending'` == False) or it will raise an exception if we go
beyond the 'timeout'.
At some point, this docstring should be reworked for clarity and, you know, whole sentences. (: