Accept Pathname object as rule's prerequisite

[gemmaro]

Hello,

This change allows the rule to accept a Pathname object as a prerequisite.

foo = Pathname("foo")

# before
rule ".o" => [".c", foo.to_s] do |t|
  # ...
end

# after
rule ".o" => ["c", foo] do |t|
  # ...
end

Thank you,

[gemmaro]

I rebased to the latest master branch. Feel free to let me know if there is anything that needs to be done to merge.