Added migrations directory.
Added make_migration and run_migrations functions.
Added redo_migration function that redoes a certain tag by doing down/up.
Migration names must be atoms.
Added some documentation regarding migrations.
Extract migration functions to boss_migrate module.  Export root_priv_dir function in boss_files.
Explain management of schema_migrations table.
Use shorter names.
Merge pull request #224 from davidw/migrations_sans_websockets

Migrations