Add index.coffee and update main for proper script loading.

This doesn't load properly using ./src/zerocater.coffee as the main entry point.

I'm on Hubot 2.3 - which is old, and I'm not sure if this is different with newer Hubot versions. Either way, many of these repos use the convention of having a separate index.coffee script that simply calls robot.loadFile on the contents of your src dir.

I see some updates in another fork - I didn't do anything to the version in package.json.