Orientation horizontal

Hello Joan,

thanks for your work on this module and especially for providing it as Open Source. 
I have been informed that you and Dr. Fischer agreed, I should add the vertical orientation features to 
your code and send a PR. 

So here it is. I think most things are clear, except for 2 things that should
be mentioned here:

1
My change of calculateOptimalWidthAndHeight, where I have commented out your 
logic, that, respecting the View's width and height tries to maintain
the aspect ratio. I found that impractical for vertical layout case, where
most of the time it will be important to fill the View's height with a page,
even if the page's original aspect ratio needs to be broken in order to
do so. Obviously one can have opinions about this. Maybe an additional
attribute on the view could help. Something like pageStretching=fitToView | keepAspectRatio

2
I have added an attribute to the View. orientation=horizontal | vertical


Let me know what you think. Thank you again,
best regards

Fabian