Readme around raise_signal_exceptions option

Allow increased visiblity around raise_signal_exceptions option which isn't obviously documented. This is helpful particular for those using Heroku which uses SIGTERMs quite liberally.
configurable default log level
Merge remote-tracking branch 'origin/pr/653'

Conflicts:
	lib/delayed/worker.rb
	spec/worker_spec.rb
Merge remote-tracking branch 'origin/pr/632'
Update wording on signal handling
Logging within signal handlers is weird

Thanks @christophermanning