Correctly adding params to multi-part upload syncs

Previously, multi-part upload options were being erroneously added to
TransferInterface objects rather than the UploadBuilder. This meant that
options were being applied to UploadPart operations rather than the
Initiate multi-part upload operation. This change moves the addition of
custom parameters from the BEFORE_TRANSFER event to the BEFORE_MULTIPART_UPLOAD
event so that they are properly applied. Closes #195.