Added support for before(:all) and after(:all)

Modified ```FakeFS::SpecHelpers``` to have additional module: ```FakeFS::SpecHelpers::All```. Including ```FakeFS::SpecHelpers::All``` will turn on ```FakeFS``` once before all your examples and then at the end of all your examples will turn it off. Modified comments in the spec_helpers.rb file to reflect the new change. Refactored code in spec_helpers.rb.