Refine internal plugin model

[ehmatthes]

Picks up from #325.

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

[ehmatthes]

Working notes

• 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.
• Get access to all sd resources through sd object, not through imports.
  • deploy_messages
  • SimpleDeployCommandError
  • sd_utils
  • fly
  • platform_sh
  • heroku
• Make any remaining imports in deployer modules relative, not dependent on being inside simple_deploy.
• Is there a better way to get platform name in sd_utils.write_file_from_template()?
  • Remove diagnostiics.
  • Set path to templates in __init__().
  • Update platform.sh
  • Update heroku
• Fly should write over files, rather than leave them as is.

[ehmatthes]

Verify hook implementations

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