Modified FacebookPublishing Plugin to upload exposure date information a...

Hi,

I have made some minor modifications on the Facebook Publishing Plugin:

1. A field 'message' (check GraphApi reference) is included in the request containing the EXIV parameters 'comment'. This publishes the image comment on the Facebook image description.
2. A field 'backdated_time' is also included with the exposure time obtained from the publishable object. This sets the image date to the exposure date on the Facebook Timeline.
I applied these changes (check the patch file attached) to the latest git commit (b1fb4bd09), compiled the code and tested it. It works as expected.

I hope you can merge them and make this feature available in the next release. It is going to be a great advantage to have Shotwell doing a decent Facebook export, given that this is hard to find, specially in an open source image gallery.