Added in the ability to disable automatic validation when a model is sav...

We like the Mongoose validation functionality. However, we have found a requirement that we need to be able to save a certain type of document even though it failed validation, because we store the validation errors in the database with that document, and a human later reviews said document.

I don't want to abandon Mongoose validation entirely just because of this requirement though. So we made a change to allow users to disable the automatic validation when saving using an option on the schema (of course its enabled by default.). 

Please merge, since otherwise we will be required to maintain our own version of mongoose.