Fix for shutter speed values from EXIF data

Shutter speeds were not displayed correctly, depending on their value (see https://github.com/Automattic/jetpack/issues/692). With this fix, shutter speeds up to 1/10000 s should be displayed correctly. Also now allows for one digit when shutter speed is greater than 1 s.
Not tested for performance issues on mobile terminals.
It would be awesome to document the `Math.abs(df-d) > tol` condition too.