ZSH and OS X compatibility

[g5pw]

Hello!
This pull request addresses some minor issues with the shell_shortcuts.sh.

• I am on OS X and use GNU coreutils, so I have GNU ls and readlink. I updated the code to check the actual commands (like, say, duck typing) and not architecture.
• the ls_with_file_shortcuts function was failing due on ZSH due to missing sh_word_split option
• ls_with_file_shortcuts was failing on ZSH due to eval interpreting the $_ll_command as a whole command, and of course a file named "ls -G" was not found in path :)

Please feel free to comment/amend my code as you will.

[ndbroadbent]

Thanks for this! That was very helpful, especially the idea to test the commands instead of the OS.
I've merged your pull request and have amended it a bit, as well as adding some more comprehensive tests for the ls_with_file_shortcuts function.
One note is that readlink does exist on OS X, but doesn't accept the -f flag, so I've changed the test to reflect that.
Thanks again!