(FACT-234) Partitions Fact

This fact adds a structured fact that reveals details of the partitions on the machine including things like UUID, size, mount point and filesystem.

This PR supersedes PR #560
Hi @adrienthebo 

I tried this, but when I run the tests, this test:
    it "should not exist when kernel isn't Linux" do
      Facter.fact(:kernel).stubs(:value).returns("SunOS")
      Facter.fact(:partitions).should == nil
    end

fails, with Facter.fact(:partitions) having a value that looks like a fact object of some sort.  Stubbing the Facter.fact(:kernel)... to return non-linux is not having any effect.

Any pointers?

Thanks,
Chris