Add System enum to give more detailed hardware/os information

[shadycuz]

The Platform class now returns a System enum instead of a String like UNIX or WINDOWS. The System enum now provides things like the architecture, distribution name and shell.

I have updated the Platform.architecture and Platform.getShell methods to use the new System enum. There is still a lot of overlap between the two and I'm not sure what I will do in the future. It seems like right now, Platform's only real use is to construct the System enum and to set the default shell. it is likely in the future I remove Platform and move it's responsibilities to jenkins.Instance or jenkins.Build.

Another change was instead of getting the architecture from uname, I'm getting it from java os.arch property.