Stopped zipping from filesystem in transform_trailheads()
Stopped zipping from filesystem in upload_trailheads()
Stopped zipping from filesystem in transform_segments()
Stopped zipping from filesystem in upload()
Removed no-longer needed compress() function
Replaced most instances of os.path.join() with string formats.

Originally, these tended to represent actual filesystem operations.
Now, most of them refer to paths on S3 and never touch the local filesystem,
so it's more appropriate to always use "/" and treat them like URLs.
Updated validate_upload() to stop writing to working directory