Add resume and content-length to ZipStreamer

- Add resume function
- Give content-length before streaming
- Fix zip structure
- Cleaning
- Count64 class replaced by floats to simplify the comparison operations (no issue for file size less than approximately 2^50 B, see https://github.com/fruux/sabre-dav/issues/408)

I also removed addEmptyDir function, because it was never useful in any zip software I tested : 
 - Winzip (Windows)
 - Winrar (Windows)
 - Windows explorer (Windows)
 - PeaZip (Windows)
 - A CSharp zip archiver (Windows)
 - The Expander (Mac OS X)
 - Unarchiver (Mac OS X)
 - unzip (Linux)

I had no error for any of these softs : zip structure is OK even if the zip was created on a 32bit platform with file size greater than 8GB.