External triggering support

This pull request implements triggering interface of libdc1394 specified [here](http://damien.douxchamps.net/ieee1394/libdc1394/api/trigger/), however, only external triggering in *Mode 0* has been verified (on Imaging Source DFK 21BF618.H cameras).

**Known issue**: driver hangs in case external triggering is ON, but no triggering signal supplied. That is most probably due to the blocking dequeueing method used in `Camera1394::readData()`, i.e. `DC1394_CAPTURE_POLICY_WAIT`.