[precompile] Import extconf.rb products from OS X stock Rubies.

* MRI 1.8.7 for OS X 10.8
* MRI 2.0.0 for OS X 10.9
[precompile] Symlink ext source to osx dirs.
[precompile] Rename prebuilt dir based on ruby platform and version.
[precompile] Add/update rake tasks and include prebuilt bins in gem.
[precompile] From extconf check if we have prebuilt bins for current platform and ruby version.
[precompile] Ensure prebuilt bins are only used on system rubies.
[precompile] Fallback require of ext to prebuilt version.
[precompile] Rename 10.9 dir to proper RUBY_PLATFORM.
[precompile] Try to fix extconf using ruby/st.h on MRI 1.8.7
[precompile] Since I can't figure out why extconf uses ruby/st.h on 1.8.7, just manually hack the header.
[CHANGELOG] Add entry about prebuilt binaries for OS X stock Rubies.

Closes #88.
Merge branch 'precompile-osx-rubies'