if linux, then xsel, else pbcopy

I successfully used this script on my Mac, but it failed to work on my Ubuntu laptop.

Now, `remotecopyserver` will check the operating system and use `xsel` on
linux and `pbcopy` on Mac. I tested it and it works with my Ubuntu setup.
Merge pull request #5 from slowkow/patch-1

if linux, then xsel, else pbcopy