Refine internal plugin model

[ehmatthes]

Picks up from #325.

• [x] Break deploy modules into small hook implementation modules, with separate module for PlatformDeployer class.
  • [x] fly
  • [x] platform_sh
  • [x] heroku
• [x] Function to get bool for whether automate_all is supported, and confirm_automate_all message.
  • [x] fly
  • [x] platform_sh
  • [x] heroku
• [x] Verify that required hook implementations are provided by plugin.
• [x] Merge
• [x] Release
• [ ] Move tests to platform-specific subdirs.
• [ ] Update tests.

[ehmatthes]

Working notes

• [x] Move logic for checking automate_all to a helper method in simple_deploy.py. (All of this should happen in existing _confirm_automate_all() method.
• [x] Get access to all sd resources through sd object, not through imports.
  • [x] deploy_messages
  • [x] SimpleDeployCommandError
  • [x] sd_utils
  • [x] fly
  • [x] platform_sh
  • [x] heroku
• [x] Make any remaining imports in deployer modules relative, not dependent on being inside simple_deploy.
• [x] Is there a better way to get platform name in sd_utils.write_file_from_template()?
  • [x] Remove diagnostiics.
  • [x] Set path to templates in __init__().
  • [x] Update platform.sh
  • [x] Update heroku
• [ ] Fly should write over files, rather than leave them as is.

[ehmatthes]

Verify hook implementations

• [x] Are they all required by default?
• [x] Is there a built-in approach to validation?
• [x] Can I get implemented hooks by inspection of pm?