Change app init protocol

[jace]

Coaster and hgapp currently require an app to be loaded and initialised in separate stages. The latter step has been a recurring source of conflict with the Flask extension ecosystem, which do not recognise a separate init step.

We should follow the crowd and switch. hasgeek/hgapp#6 documents the changes. Coaster will require some breaking changes to make this happen, and all apps will need to be updated. In recognition of this change, Coaster's version number will be bumped up to 0.6. Specifically:

• coaster.manage will no longer accept env as a command line parameter.
• coaster.manage will no longer accept init_for as an init parameter, nor will it be called.
• coaster.app.init_app takes env as an optional parameter, reading it from the FLASK_ENV environment variable if unspecified, and defaulting to DEVELOPMENT.