Fix Dir.foreach with relative paths.

Dir#read gets contents that are the full path (e.g. /path/to/file.txt) and then strips off the path of the directory (e.g. /path/to) before returning them (e.g. file.txt). This works fine when Dir#path is absolute, but not when it is relative. For example, this should print out "file.txt" but it was previously printing "/path/file.txt":

  FileUtils.mkdir_p("/path/to")
  File.open("/path/to/file.txt", "w") {|f| f << '' }
  Dir.chdir("/path") do
    Dir.foreach("to") do |file|
      puts file
    end
  end

The fix was to always store the normalized path as the Dir path when it is initialized. In addition I changed it to stop using the dubious gsub in favor of something that actually checks that it starts with the path.
Merge pull request #208 from eventualbuddha/fix-foreach-with-relative-paths

Fix Dir.foreach with relative paths.