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")# beforerule".o"=>[".c",foo.to_s]do |t|
# ...end# afterrule".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.

[hsbt]

@gemmaro Can you revert 27d0e0e? Renaming is out of scope at this feature request.

[gemmaro]

@hsbt Sure, I dropped the renaming commit.