allow spaces in PATH elements

I have the following in my `PATH`: `/Applications/Android Studio.app/sdk/platform-tools`. Perlbrew 0.71 dealt with this just fine, but 0.72 chokes on the space in "Android Studio.app" and truncates the path at this point.

A judicious use of quotes fixes this.

There are no tests for the handling of weirdnesses in `PATH`, and I was unsure where best to add them. I've left that alone.