load the image when the slide is active

[bogomips]

This is a proposal for the issue #40

On a further update, in order to implement the preloading of the "next" picture, we can use the ion-slide events ionSlidePrevEnd, ionSlideNextEnd instead of ionSlideDidChange.
In this way the code would know if to preload the slide+1 or slide-1

[bogomips]

The second serie of commits adds the capability of requesting images with an extra header, useful for example to include jwt token in order to get the capability of displaying images that require authentication to be served.