Adding initial spec and class for Message/SlashCommand
Adding static create() method to Phlack\Phlack
Ensuring error responses are returned instead of Exception being thrown
Updating README.md

Adding some basic documentation
Merge branch 'feature/commands' into develop

* feature/commands:
  Updating README.md
  Ensuring error responses are returned instead of Exception being thrown
  Adding static create() method to Phlack\Phlack
  Adding initial spec and class for Message/SlashCommand