Add zsh support

[abend]

Hi. Thanks for this great package!

I had trouble running the tests using zsh. It appears to be a difference in how bash and zsh handle single quotes inside single quoted strings. I think bash is a little more lenient.

This fix allows the tests to pass under zsh, bash, and sh. Csh and ksh still have problems, but since I don't use them, I haven't put any effort into them.

If you like, I can add a feature that runs the tests under different shells (if installed). I wanted to keep this patch small though.

Let me know if you'd like anything changed.

Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling 282497a on abend:feature/zsh-support into 590773b on jacktasia:master.

[jacktasia]

Awesome, thanks for doing this!