Remove the unused thread pools

We now rely on RMQ's thread pools for delivery, and our own were unused. Also remove tests and examples that let you pass in thread pools since they are outdated.
Resurrect the :executor_factory option

Rewrite the test to assert that the factory is used.
Remove nested converting_rjc_exceptions_to_ruby
Merge pull request #24 from ruby-amqp/remove_thread_pools

Remove the unused thread pools