Validations not enabled by default

[lawso017]

@p8 Here is a PR that changes the validations so they are all disabled by default.

I think I hit all the Readme docs as well to note that validations are optional but can be enabled either globally (via the config file) or in individual models when mounting the uploader.

Again, my argument for this is that version 0.0.15 did not have validations. So code running & tested under that version should not break due to the presence of new default validations during an upgrade. But wanted your take on it as well so I've done it on my branch first.

[p8]

@lawso017 Sorry for the confusion, but 0.0.15 did have validations (https://github.com/dwilkie/carrierwave_direct/blob/v0.0.15/lib/carrierwave_direct/validations/active_model.rb). Things probably broke because of the multi column implementation. I agree we might want to turn off default validations but this might break backwards compatibility for those who expect the validations to be turned on.