Refactored install procedure and other small improvements.

Hi, I haven't yet terminated all the editing, but i'm publishing this pull request so you can have a look at my changes while I complete the last details.

Basically what I did can be summarized in those points:

+ Refactored install procedure, now it creates a Migration for easy rollback or user customizations.
+ Included support for Foreign Keys, enabled by default but can be disabled while installing.
+ Removed ``acl:reset`` in favour of ``acl:update reset``

*TODO*: Update the Tests if necessary and the README with the new install procedure.


*Footnote: I also have some more improvements to this nice library but I will publish them in separate branches once ready.*