Lots of EXIF mode fixes and improvements by Dennis Real (closes #87)

Squashed commit of the following:

commit 58dd2e30
Merge: cf6f0320 82a976e0
Author: Dennis Real <github@tildepipe.org>
Date:   Sat Mar 31 18:43:47 2012 +0200

    Merge branch 'master' of git://github.com/derf/feh

commit cf6f0320
Merge: 10b7b1e3 877e6346
Author: Dennis Real <github@tildepipe.org>
Date:   Sun Mar 25 19:07:30 2012 +0200

    Merge branch 'master' of git://github.com/derf/feh

commit 10b7b1e3
Author: Dennis Real <github@tildepipe.org>
Date:   Sat Mar 24 14:41:15 2012 +0100

    Support for Canon Exif makernote tags

commit 3c284dc8
Author: Dennis Real <github@tildepipe.org>
Date:   Thu Mar 22 19:44:32 2012 +0100

    Support for Active D-Lightning

commit ad5aea8c
Merge: cc6de200 9c43b648
Author: Dennis Real <github@tildepipe.org>
Date:   Thu Mar 22 19:01:38 2012 +0100

 ...