Further separate the example into four sub projects

[lastland]

There are now four sub projects:

• app: the real application. this depends on the generated_code sub project.
• migrations: the migrations and code generators are defined here. this is also where most commands are run. this project depends on the generated_code sub project.
• migration_manager: where user customize their migration manager. this project does not depend on any other sub projects, thus can be used to run "rescue" commands when, for example, generated_code is broken.
• generated_code: where generated code are stored. users are never supposed to edit this project manually.

There are currently two sets of commands: one starts with mg, like mg preview, mg apply; the other starts with mgm, like mgm rescue, which will delete all generated code. NOTICE: the commands are not yet well separated into mg and mgm, so any suggestions on how to separate the commands are welcome.

I didn't use an sbt plugin because sbt will load a user's customized migration manager on starting, thus a reload is required every time the manager has been changed. I imagine there will be a "migration_manager" sub project in every user's project by default, even when users don't have the desire to customize it.

[cvogt]

read it again. What can I say. LGTM. I like how you think :)

[cvogt]

merge it :)