Forcing the RSpec version to >= 2.14

New expectations require `yield_control.exactly` which was added in
RSpec 2.14.
Adding Dir.foreach (ref #4)
Adding a :compare rake task

Calling `rake compare CLASS=Dir` will display the list of original Ruby
methods missing from MemFs implemantation and the list of methods
present in MemFs and not in Ruby.
Adding Dir.new (ref #4)
Adding Dir#each (ref #4)