Made Content-Type a configuration-level option to better fix issue #63.

I still needed the Content-Type option, but I understand that if you include Content-Type in your
policy document, but don't include the corresponding field in your form, S3 will return 403's.

A better solution is to make it a configuration level option. I also added documentation
to clearly specify that you will have to add a Content-Type field to your form.
Add :will_include_content_type to CarrierWave configuration.
Add @colinyoung to contributors list