Set readability-identifier-naming in clang-tidy config

http://clang.llvm.org/extra/clang-tidy/checks/readability-identifier-naming.html

https://github.com/llvm-mirror/clang-tools-extra/blob/master/test/clang-tidy/readability-identifier-naming.cpp

https://github.com/llvm-mirror/clang-tools-extra/blob/master/clang-tidy/readability/IdentifierNamingCheck.cpp#L67