Correcly display the Aperture

I always get the same Aperture value in the right menu: "Aperture 1/1".

This is because of 2 bugs:
- PhotoShow use EXIF ApertureValue instead of FNumber
- the code is missing parenthesis: ``ìf (a = fct() > 0)`` should be ``if
  ((a = fct()) > 0)`` or a will be affected to the test ``fct() > 0`` so
  1 (or true).

For a sample image I have the EXIF data (returned by ``exif_read_data()``):
- COMPUTED.ApertureFNumber: f/10.0
- EXIF.FNumber: 10/1
- EXIF.ApertureValue: 6643856/1000000

The easiest would be to use the COMPUTED.ApertureFNumber value
returned by ``exif_read_data()`` but I did not succeed (I do not know PHP).
So my patch uses EXIF.FNumber and changes the display to be "f/10.0"
Merge pull request #2 from LudovicRousseau/master

Correcly display the Aperture