using numeric expiration value to make RabbitMQ (dev.rabbitmq.com) happy

This fixes error:

server channel error 406, message: PRECONDITION_FAILED - invalid expiration 'later': no_integer

The specification says that "expiration" is shortstr but dev.rabbitmq.com expects an integer.
Merge pull request #24 from viliampucik/master

Numeric expiration value to make RabbitMQ (dev.rabbitmq.com) happy
Split rabbitmq-c and Net::RabbitMQ.