Add Support for Auto-Populating Description in Version Changes

[zmievsa]

This is a great style of doing version changes.

class MyVersionChangeVersionChange):
    """ABC"""

    description = __doc__
    instructions_to_migrate_to_previous_version = ( )

We should support it natively like so:

class MyVersionChangeVersionChange):
    """ABC"""
    instructions_to_migrate_to_previous_version = ( )

So cadwyn will automatically use ABC as a description.

However, the description should still be non-empty and it must be populated by the user.