Improve readability of Backup()

[codesoap]

When reading about the function on godoc.org, I was confused about which connection was the destination and which the source and had to look at the code to find out.

[coveralls]

Coverage remained the same at 50.364% when pulling 17bc447 on codesoap:master into c64f703 on mattn:master.

[mattn]

Thank you