List argument for FileUtils.mkdir and FileUtils.mkdir_p

According to the [Ruby Doc](http://www.ruby-doc.org/stdlib-1.8.7/libdoc/fileutils/rdoc/FileUtils.html#method-c-mkdir), `mkdir` and `mkdir_p` can also take a list of directories as argument. I updated the fake methods to handle this (inspired from the existing code in `rmdir`).