(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
These tests cover both the fact as well as the `Facter::Util::Partitions` code; the tests for the fact should just test that the fact itself reads the data from the partitions code correctly and tests for the partitions methods themselves should be broken out. It will reduce the amount of stubbing that needs to be done in this `before` block.