Update the API at the date of today (commit date)

Changes:
- Update all the method with https://api.slack.com/methods at the date of today
- BREAKING CHANGE: methods are now defined in the Client class rather than use
  the hacky #missing_method
- Provide a testing utility (needs to add a slack token)
- Fix gem dependency (rake was missing)

As the gem works well, I bumped the version to 1.0. I hope you don't mind this jump. ;)