Use migration fields in migration methods

Changed migration create_translation_table method, to use fields in the migration instead translation fields write in the model file. 


This solves some problems with migrations, for example, when you add a new translated field in a second later migration: if you run all migrations in a clean database, the first translation migration will create the 2 translation fields because in that moment the two fields are being translated in the model. This way the second migration would crash as it would try to add  one existing column.

This is related with issues #98 and #131