Add functions to normalize Winodws & Unix paths

The purpose of these functions is to be able to join file/dir paths safely without trailing slashes, basically for the same reason as normalize_uri.  Some modules are really buggy when merging paths, so instead of letting them do it, it's better to use these functions.

To test these functions, simply do:

	$ rspec file_utils_spec.rb 
	.........

	Finished in 0.00207 seconds
	9 examples, 0 failures