Skip gdb signal handlers in core stacktrace generation

Closes #99

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>
Return preselected crash thread of core stacktrace

Closes rhbz#997076

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>