Allow double-quotes in commands on Mac.

Allows double-quotes in commands on Mac OS X. Currently using double-quotes results in an unfriendly error message. Quotes in commands are useful when setting up remote shells. For instance, here's my present configuration:

```
---
  commands:
    - name: i3server
      command: ssh -t ubuntu.dev.sys 'bash -ilc " cd app-ido-i3; source develop; devctl start; paver run "'
```
A similar correction should be made for single-quotes on Linux, but I'll let you handle that as I don't have the facilities to test it.