allow building with the system jsoncpp instead of the bundled one

now we can build with the jsoncpp of the system by passing
 -DUSE_BUNDLED_JSONCPP=OFF
or even use an own jsoncpp with
 -DUSE_BUNDLED_JSONCPP=OFF -DJSONCPP_PREFIX=/opt/superjson

for backwards compatibility, build the bundled version by default
Merge pull request #72 from evgeni/system-jsoncpp

allow building with the system jsoncpp instead of the bundled one