Added Hook 'beforeValidation' and updated docs

Added a hook called `beforeValidation` to address #155 as well as updated docs to reflect the addition.

I also changed the `package.json` file to update sqlite3 from version 2.1.5 to 2.1.7. Version 2.1.5 prevented me from compiling the module during `npm install` whereas 2.1.7 completed successfully.  Hopefully it will still be backwards compatible.