[Closes #63] Migrations

A `migrations_dir` configuration parameter was added which indicates where `sumo` should look for migrations `*.erl` files, it's default value is `src/migrations`. Migration modules should be named with the following format: `yyyymmddhhMMss_description_with_undescores`. The most recent migration will be considered the one with the most recent `year-month-day-hour-min-sec` value.

In order for migrations to work in a given project, `sumo_migration` should be added to the `docs` list in `sumo`'s configuration.

The following targets can be added to the project's `Makefile`:

```Makefile
migrate:
    erl -pa ebin -pa deps/*/ebin -s your_app -s sumo migrate -config path/to/config

rollback:
    erl -pa ebin -pa deps/*/ebin -s your_app -s sumo rollback -config path/to/config
```

 If your `migrations` folder is not under `src` the you should make sure to compile them and include the path to the compiled files in the call to `erl`. 
Why are those functions required to return a ``string()``?
I think ``ok`` is a better choice.