added LLDB and GDB class handlers to obtain stack traces. LLDB for Darwin, GDB for Linux. refs #3616
I found a case when lldb has exited while I was still trying to send commands to it. refs #3616
Merge pull request #3617 from milljm/memory_logger-lldb

added LLDB and GDB class handlers to obtain stack traces
Merge commit '08914a0fb55b693633e28e2437289b2aa4a609ea'