First check for pbcopy, then for xclip

Some Mac users may have xclip installed, and there are some cases where
xclip isn't working properly, so we just ensure here that Mac users will
always use pbcopy, and Linux users xclip.

Fixes #19.
Merge pull request #20 from janko-m/yank-xclip-issue

First check for pbcopy, then for xclip