Yeoman generator

This deprecates the default generator using `hubot --create` in favor of https://github.com/technicalpickles/generator-hubot

I originally set out to fork @patcon's https://github.com/technicalpickles/generator-hubot and https://github.com/patcon/generator-hubot-script, but ended working through the various yeoman tutorials, and the styles ended up quite different. Between that and wanting to add a bit more functionality to the bot generator, and using namesapces, I ended up with  https://github.com/technicalpickles/generator-hubot 

In particular, I've removed basically all the generated scripts in favor of external ones. That has been a serious maintenance burden burden and point of friction for new users. I've moved those all to external scripts on the hubot-scripts org.

I intend to move that generator either to the github org or hubot-scripts org, haven't decided which. A minor thing I think I'd like to see is instead of `yo hubot:script`, make it explicit about `yo hubot:external_script` versus a `yo hubot:local_script` for generating a package versus a script inside the hubot directory you are in.

I also hadn't decided how to handle the heroko support. I had talked to @tombell at some point about including a script like `hubot-heroku-keepalive` that would be look for a HEROKU_URL like your heroku remote, create a HTTP endpoint, and handle setting up a `setInterval` to ping that endpoint ever whatever seconds. That's kindof a nice-to-have though.

Thoughts?

cc @jjasghar @therealklanni @alindeman @ymendel