Added #save! for more Rails-ness (and gems like FactoryGirl)

Like ActiveRecord's #save!, this method tries to save and raises an error if it fails. ActiveRecord raises RecordInvalid, and Her now raises Her::Errors::ResourceInvalid.

As well as bringing Her more into line with ActiveRecord, this is particularly useful if (like I am) you're trying to use gems such as FactoryGirl which rely on this functionality.