use json.h from the include path instead of hardcoding third-party

this allows using the proper header files while linking to the system
jsoncpp when using -DUSE_BUNDLED_JSONCPP=NO
When compiling with an external jsoncpp, the embedded version should not be used whatsoever.
Merge pull request #175 from evgeni/no-third-party-jsoncpp

use json.h from the include path instead of hardcoding third-party
ga tracking added
ga tracking added pixel image
Support EL7
indentation