fix support for spaces in test command arguments

Use `bash -c "$CMD"` to handle argument parsing rather than doing it manually
add win32 support for spaces in test command

This should fix the code in the previous commit to support win32 platforms.
Based on code from: https://github.com/joyent/node/blob/v0.11.13/lib/child_process.js#L599
Also, switched from `bash` to `/bin/sh` to be even more universal for unix-based platforms.
rm extra call to spawn and minor cleanups
fix typo for spawning test commands on unix
Merge pull request #37 from KylePDavis/patch-1

fix support for spaces in test command arguments