Revise Promise support about intercom-node

[bobjflong]

I'm not opposed to falling back to promises if a callback is absent. Should be easy and doesn't break compatibility afaik. But - I'd point out that the "core" code that switches between callbacks and promises is quite small and localised (client.js) - so I'm not sure there's much benefit in redesigning it to use promises only + nodeify.

We'll definitely always support callbacks I think - if for no other reason than backwards compatibility.

[tejasmanohar]

@bobjflong Alright. I see what you're saying. I'll look more into that sometime next week and see if can be cleaned up some to use nodeify... and if so, send a PR.

[Nathan219]

When using the usePromises(), I get warnings when I get errors

Warning: a promise was rejected with a non-error: [object Object]
    at resolver (/Users/nathan/runnable/api/node_modules/intercom-client/dist/client.js:115:19)
    at callback (/Users/nathan/runnable/api/node_modules/intercom-client/dist/client.js:175:11)

We're using node 10.26, with bluebird. If I promisifyAll on the models, I get acceptable errors. But when using usePromises(), I can't even tell what the error is at all

[bobjflong]

@Nathan219 thanks for raising this. Would you mind checking out this branch? #64

I plan on removing usePromises and falling back to promises if no callback is provided.