Test on Mac OS

[AldaronLau]

System Support Request

Make sure that the library works on Mac OS.

• Fix Getting OS Name
• Fix Getting Computer Name
• Fix Getting Environment

[cygnus9]

I just had someone test it on a mac, and at least it compiles now. Getting the username also works. Getting the OS or computer name fails at runtime and getting the env simply returns "unknown".

[AldaronLau]

Next time I use a Mac, I'll try and fix it.

[AldaronLau]

Update: I finally got around to testing it on a Mac and I fixed getting the OS name, computer name and environment. whoami 0.4 is out on crates.io with the changes.

[cygnus9]

Cool, thanks.