Fix path to String coercion
Allow coercion of path to string via #to_path

ruby-spec suggests this has been the behaviour of File
methods since 1.9.
Specifically Pathname implements #to_path
Work naturally with File and Pathname

Follows ruby conventional behaviour
Class method on File takes a path
Instance method on Pathname that forwards to the File method
Document extensions
Merge pull request #9 from lwoggardner/pathname

Xattr work with File and Pathname as conventional ruby