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.
add license information to gemspec

This way you can get this info when using rubygems.org API
Fix fileutils.cp when using a directory as destination and a subdirectory
as source, see https://github.com/defunkt/fakefs/issues/176
add a test for the issue of https://github.com/defunkt/fakefs/issues/176
Add File.executable?
Add File.zero?
Allow FileSystem.cd to accept a block.
Q&D fix for missing File.realpath on 1.9.3
List argument for FileUtils.mkdir and FileUtils.mkdir_p
Missing FileUtils aliases.
Fixed inconsistent remove alias test.
Merge remote-tracking branch 'AlphaHydrae/fileutils-mkdir-list'
Merge remote-tracking branch 'RyanScottLewis/file_utils_cd'
Add tests for FileUtils.cd with block (RyanScottLewis's patch @ 633df44c6baf634b6972524a4b4096746acf67e4)
add optional (but for now, ignored) flags to Dir.glob and FileUtils.cp_r

Closes #182
Merge remote-tracking branch 'emilsoman/file-zero'
make File.zero? a one-liner
Merge remote-tracking branch 'xaviershay/file-executable'
Merge remote-tracking branch 'mscherer/master'