add Grammar support for C, C++, Objective-C, Objective-C++
Add basic C, C++, Objective-C, and Objective-C++ specs
Augment Xcode based C specs
Create an OperatingSystem utility module

Define the OperatingSystem#isDarwin helper to use for C, C++,
Objective-C, and Objective-C++ code.
Remove some code duplication inside of grammar-spec

Create an @reloadGrammar function which can be called to remove the
cached grammar module and require it again.
:memo: Add notes about Mac OS X only runs with clang/clang++
:apple: Update CHANGELOG.md
Update contributors (`git shortlog -sne`)
Merge pull request #207 from rgbkrk/xcrun-c-support

Add Mac OS X C, C++, Objective-C, and Objective-C++ support (from @caodingpeng)