Use options from modified goamz
note unused code
fold args into struct
refactor prefix

Change prefix to be from bucket name to prefixing the uploaded file path.

This should allow doing

fs2s3 -b abucket -p somedir -x further/

to recursively put adir at further/adir in the bucket bucket
non code edits
tidy notes
add mime type flag

Also lexicographically sorted  some args