Add methods to pause task execution progress.

Add pause(), is_paused() and resume() methods to give ability to make
pauses during task execution. After resuming remaining time is
calculated using only milestones that were passed from the resuming
moment.
Fixing test t/serialization_api.t

Actually this work was done by wtertius@, I've just rebased it and changed a
litte to match new test code.
Fixing misspell
Method paused() dies if the object is already paused