-Wshadow

* https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
* http://programmers.stackexchange.com/questions/122608/clang-warning-flags-for-objective-c-development/124574#124574

In clang: `-Wconversion` implies `-Wshorten-64-to-32`, but we cannot use
`-Wconversion` yet.

Author: cdunn2001

CMakeLists.txt

@@ -97,10 +97,10 @@ endif( MSVC )
 
 if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   # using regular Clang or AppleClang
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wshorten-64-to-32")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wshadow -Wshorten-64-to-32")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   # using GCC
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -pedantic -Wno-long-long")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wshadow -Wextra -pedantic -Wno-long-long")
 endif()
 
 IF(JSONCPP_WITH_WARNING_AS_ERROR)