improve cleanup of connection files...
improve cleanup of connection files
* allow clean shutdown of qtconsole on sigint
* init_kernel_manager registers cleanup_connection_file with atexit
* connection_file trait is guaranteed to be abspath at end of init_connection_file(), so find methods are no longer necessary after this call.
* move cf removal from KM.__del__ to KM.cleanup_connection_file
* _new_connection_file only uses last segment of uuid, not the whole thing, which is ugly and overkill 2**48 is enough kernels.
