Slug based deploys

[apsoto]

Here's my PR for supporting slug based deploys on Heroku. It required a minor refactoring of PendingMigrationCheck to make the interface simpler so you can provide your own migration_check class.

The README has usage docs on how to use slug-based deploys and run migrations using the migration_check option.

I'm using it right now for multiple production apps, so it's got real-world usage. I'm open to further refinements, so let me know what you think.

[mattpolito]

Thank you very much for taking the time to add this feature. I'll go through it as soon as I can. I appreciate that you're able to use it already for real-world applications.

[apsoto]

just pinging to see if we can get some movement on this. Thanks.