chat.PostMessage: Switch to HTTP POST instead of GET

[BrandonBoone]

Ran into an issue recently using Noobot where I hit Slack's max URL length calling chat.postMessage. Seems this limit could be increased by switching to use an HTTP POST on the call to chat.postMessage and sending the payload as JSON which supports up to 40,000 characters in its text parameter: https://api.slack.com/methods/chat.postMessage

If you're open to PR's I may be able to make this adjustment.

[BrandonBoone]

Apologies, this looks like a dupe of #83. I'll close and pick up the conversation there.