Runtime panic

The last pull request I made unfortunately introduced a runtime slice index panic if neither xclip nor xsel were present on the client machine. This fixes that and restores the prior behaviour of just letting the *exec.Cmd fail when it attempts to run with an invalid path name.

I've also changed the flags that are presented to xclip because the tests weren't passing on my machine; in fact it looks like the -secondary flag comes from xsel and should have been -selection[0] all along.

[0] http://linux.die.net/man/1/xclip