Added support for Scientific Linux 6+ via osfamily fact.

This requires facter 6.2, which perhaps should be mentioned in the README.
http://projects.puppetlabs.com/projects/facter/wiki#New-fact-6792-Added-osfamily-fact

Also specify empty namespace for osfamily per style guide.
http://docs.puppetlabs.com/guides/style_guide.html#namespacing-variables
Updated README.md as well.
Merge pull request #3 from jbarbuto/scientific_linux_support

Added support for Scientific Linux 6+ via osfamily fact.