Split lfs_migrate command to the bunch of South migrations.

[Zagrebelin]

I propose to change custom migration command to the set of self-writen South migrations.

Pros:

1. It's impossible to migrate between minor versions (0.7.2->0.7.5).
2. We can provide forward and backward migrations to revert any update.

Cons:

1. It's hard to first time migrate. It's necessary to choose migrations that alredy done and launch manage.py migrate --fake xxx where xxxx is number of migration.

[diefenbach]

This has also been done here: https://github.com/diefenbach/django-lfs/pull/34

Thanks for contribution! Kai