add -x parameter for aria2c to speed up downloading

manpage:
> -x, --max-connection-per-server=<NUM>
>              The maximum number of connections to one server for each download.  Default: 1

increase the number can briefly help speed up the progress.
Merge pull request #61 from PeterDaveHello/patch-1

add -x parameter for aria2c to speed up downloading
Update ppa info, refer to #58
add -x parameter for aria2c in apt-fast.conf