Color parsing for BIGRGB and fixes for its conversion to bagfile.

Hi Lorenzo,

Here's my work since the last pull request.

I added code for converting the narrow image to a colored sensor_msgs::image.

Also, regarding your issue #45 with the narrow image, I found that the timestamp of the narrow camera image does change, even when no new images is in there, leading to black images in the bagfile.  
I therefore replaced the corresponding condition in parse_all_frames.cpp line 169 by a check on the sf_valid_flags. Contrary to the comments in superframe_v2.h, the field
    uint16_t sf_valid_flags[2];
indicates a new narrow image iff sf_valid_flags[1] == 7. Not sure if that's a version thing, but it works well for me.

Best,
Felix