doc fixes
Process#scheduled_work -> Process#frequency
remove the require of json
note addition of XPool::Process#frequency in ChangeLog.txt
README updates
reformat & improve ChangeLog.txt
expose Process#frequency as public API.
move _shutdown above spawn
ensure @busy is set to false in case of exception.
test the behavior of Process#busy? in case of exception
forgot to add support file
call wait inside 'ensure' incase it raises a SystemCallError
_shutdown should never raise, so simplify a bit.

In the past it could have been possible for _shutdown to raise but since we
abort calling _shutdown if the process is considered dead this should never
happen.
update dated documentation
raise a RuntimeError when pool/process is not active
clarify raise message
improve performance of the test suite
use Sleeper instead of IOWriter in test
return 'self' from Process#schedule
return an array of XPool::Process objects from broadcast