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.
Fixed typo and added example to spec_helpers_spec.
Merge remote-tracking branch 'dyba/support-before-all'