build: use latest jansson 2.7 from ppa:pi-rho/security
build: remove jansson_compat.h, require newest jansson version (2.7)
protobuf2json: implement bytes type support
test: add zero byte encoding test for bytes values
test: add test for repeated bytes value
test: explicitly cast to (const char *) to avoid warning with clang
other: add comment about include
build: return --error-exitcode for cppcheck make target