Added keep-alive agent

Added the keep-alive agent to keep the connections to the service bus open between requests. This significantly reduces the round-trip time for adding a message to the queue.

A more elegant solution would be to expose the agent, so that you could use any agent you see fit.

This has been discussed in the following issue: https://github.com/Azure/azure-sdk-for-node/issues/1338
Could you make this a property on the client and set it as default to secureKeepAlive and then just use that value here ?