Fix latest implementation
Add additional comments to each provider method

To help clarify the logic, add a mostly-standard format header to each
method in the provider.
Provide a File::NULL fallback for Ruby 1.8.7

Using the File::NULL constant allows for platform-independent code
(read: will work on Windows) but File::NULL was only introduced in Ruby
1.9.3. This commit introduces a posix-only fallback mechanism to let
ensure=latest behavior work under Ruby 1.8.7 on posix platforms.