Update lib/s3_direct_upload/form_helper.rb

Make bucket region configurable
Update README.md

Add region prefix doc to README
Update lib/s3_direct_upload/config_aws.rb
Update lib/s3_direct_upload/form_helper.rb
Update lib/s3_direct_upload/form_helper.rb
Merge pull request #42 from reinkcar/master

Add region to config