Limit the number of Channel instances
Make variables final
Add code to initiate a new Channel
Add some basic checks of the created Channel
Document getChannel
Add some checks to getChannel as well
Move props file keys to constants

This changes some of the keys, but there are no compatability issues yet
so it should be fine. Also note that ackssk was left out deliberately
since that code will be deleted/rewritten anyway and will use the
private key.
Move next slot calculation to new method
Add method for sending a message
Add the header to the message
Include a real id in the messages
Refactor so more message types can be accepted
Document the new method
Check the parameters
Allow message to be null if there is no data
Check parameters in sendMessage()
Check for null parameters in other public functions
Add the skeleton for more of the Channel API
Refer to messages using UID instead of message-id
Initialize the message before adding headers