Merge pull request #4 from pwithnall/sigint

Print alive objects on SIGINT, SIGTERM, SIGABRT and SIGSEGV
Support outputting additional debug information for certain GTypes

It is useful to see at-a-glance debug data for certain types of GObject
in the dumped object list. Initially, this feature supports printing the
source tag of GTasks, but other GObjects could get special treatment in
future.

To be useful, this requires the program being debugged to be compiled
with -rdynamic.