(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
`Facter.fact(:partitions)` is the object that can generate values; this previously worked because the fact itself was being conditionally generated. You want to do `Facter.fact(:partitions).value.should be_nil`.