Improvements for next release: adding additional getters

Just a few backward-compatible changes:

 1. `is_connected` field added to var_dump output for `AMQPConnection` and `AMQPChannel` objects

 2. `AMQPChannel::getConnection()`, `AMQPQueue::getChannel()` and `AMQPExchange::getChannel()` methods added to retrieve underlying `AMQPConnection` and `AMQPChannel` respectively.

The reason for adding these method is to allow to change runtime settings on `AMQPConnection` and `AMQPChannel` from  `AMQPChannel`, `AMQPQueue` and `AMQPExchange`.

For example, we may have `read_timeout = 10`, but when we are consuming messages we have to set `read_timeout=0` to listed endless for new messages.

Sure, we can extend core classes and store `AMQPConnection` and `AMQPChannel` there but why should we do that when they are already stored but has no successor?