Fixed a bug in the fdwalk method where it was not honoring lowfd, and added a test for it.
Matz Ruby 1.9.3 compatibility for fdwalk and closefrom

Matz Ruby 1.9.3 reserves file descriptors for internal
thread/signal handling that cannot be safely closed[1],
so we use the new C API function (rb_reserved_fd_p())
to skip reserved descriptors.

ref: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/37759

[1] - both descriptors have the close-on-exec flag set, however
Added changes so far for 1.2.7.
Version bump in the gemspec, and updated the test-unit dev dependency.
Uses F_NOCACHE for macos
Fixed bugs
Merge pull request #2 from genki/master

Uses F_NOCACHE for directio on MacOS
No longer omit directio tests on darwin.
Added note about directio to CHANGES file.
Updated notes for OSX, copyright and homepage info in the README.
Updated test-unit require line in test file.
Check for rb_thread_call_without_gvl function.
Use rb_thread_call_without_gvl where available.
Updated gem:create task for Rubygems 2.x.
Added note regarding rb_thread_call_without_gvl.
Bump test-unit dev dependency to 2.5.0 and simply check for MS Windows.
Added a check for HAVE_RB_THREAD_CALL_WITHOUT_GVL for 2.2.
Merged from master.