[FEATURE] Process consolidation into a single process

Now a user can run the entire gatewayd stack in a single node.js process with:

    bin/gateway start

Alternatively to start the gatewayd processes individually in their own system process run the following:

    bin/gateway ps:start outgoing
    bin/gateway ps:start incoming
    bin/gateway ps:start ripple_rest
    bin/gateway ps:start server

Every file in /processes exports a function that takes a single argument, the gatewayd object. Each of these isolated functional processes can therefore be required by other node.js processes and started as a long-running procedure.
Can we spell `err` out?