1) Added references mechanics to Fields, for use in ABMCombinado.

2) Validation bugfixes, and extra checks in inner methods.
Implemented a first stable and functional version of ABMCombinado, which mixes an arbitrary ammount of ABMs and can reference fields between one and another. For example, you can create an ABMCombinado for "person" and "user" at the same time, and reference the "id_person" field in "user" table to the "id" of the "person" table. That way, when the person's id changes (as in when it is created, from none to a brand new id), it also changes the referred field. This makes very easy to mix ABMs.