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
Fixing test on perl 5.8.0
Explicidly specified that Perl 5.8.0 and above is supported
Created CONTRIBUTORS section and added Dmitry Lukiyanchuk
Added testing method is_paused()
Getting 100% branch test coverage
Added failing test, showing current problem in the implementation
Fixed method get_elapsed_seconds() when using pause()
Removed pause testing from the main test file

Reason: the code bacame too complex.
Correct work of get_remaining_time() after resume()
Renamed `_paused` to `_is_paused`
Fixing a bug that it was not possible to spawn a paused object
Fixes in serialization test
Fixes in POD
Added tests to get 100% branch test coverage