fix: double announcements on the channels

- As per [Shaun's recommendation](https://github.com/shaunburdick/slack-ooo/pull/15#issuecomment-247882697)
- Added a little styling on the OOO message

What if someone puts in the config a channel that does not exist?
Or mispells it? Or the channel is renamed?

This is a minor fix to only loop over valid channels.

Leter on, might be good to raise a warning on the console.

This was being caused by the nested `forEach` loops in bot.js
[here](https://github.com/shaunburdick/slack-ooo/blob/cd6a4e59734f8ee9779d59f37a9f1b2c4126caad/lib/bot.js#L192-L208)

* Adding announcement feature from #5
* Added announcement loop and tested it all out
* Added more details about announce feature in README

* Rewriting _everything_
* Adding travis yml file

Better lang processing

#3: Added chrono for date parsing

Added some bug fixes and resolves #2