0.15.0

This addresses a few small issues #146, #147, and #148.
The most important changes are:

1. Added "-fexceptions" flag to subprojects/js/CMakeLists.txt, which I forgot to do.
2. Added a function "set_message_level()" to ojph_message.h, which controls the severity level at which info/warn/error messages are printed. By adjusting the message level, users can tailor the verbosity of the output to their specific needs.