(CFACT-27) Add virtual and is_virtual facts for OSX VMs.

Adding virtual and is_virtual facts for OSX virtual machines.
Detects VMware, VirtualBox, and Parallels.
(maint) Add OSX and Ubuntu dependency install instructions to README.

Adding a section to install cfacter dependencies via homebrew for OSX.
Adding a section to install cfacter dependencies via apt for Ubuntu.
(CFACT-27) Add virtual and is_virtual facts for Linux.

Adding detection of many different virtual machine hypervisors under
Linux.
Merge pull request #53 from peterhuene/facts/virtual

(CFACT-27) Add virtual and is_virtual facts for OSX and Linux.