Make the max upload size an overridable option.

This lets a specific request bypass the global config for a max
file size.
Enable File#upload_request to pass model options

If you have a model User with an :avatar s3 file, you can
define User#avatar_max_upload_size to set a default max upload
size just for this model
Clean up the File option/default option code

It's more obvious if we do a merge in the intializer with defalts
rather than have the whole setup* call
Merge branch 'bem-5_max_upload_size'
Version bump for release.

Added new max_upload_size to api so bumping.