(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
Ruby 1.8.7 doesn't have `Hash#reject!`:

```
└> rbenv run 1.8.7-p371 pry
[1] pry(main)> Hash.respond_to? :reject!
=> false
```