Allow for boolean values in tables

I tried to build something like `JSON::XS::Bool`. Basically, I provide two simple constants `true` and `false` on `Net::AMQP::Common` which can be used to set true/false on a table value. They evaluate to true or false, but are refs which lets us pack them specifically as booleans.

This is to fix https://github.com/ewaters/net-amqp/issues/9