Content view position + AppleDoc

In most of the cases you want to see the modal view in the center, but in some cases you want it to be somewhere else. For example if the view has a textfield, you want it to be in the top, so the keyboard doesn't cover it.
For that purpose I have added properties to define the alignment (horizontal+vertical) of the view. By default it will be centered, but you can change it by just changing the values of alignment and spacing.

Also, I have added comments in the .h file, just to suppress the warnings from the AppleDoc compiler. The code is self explanatory I think, but AppleDoc is a good feature to have in XCode anyway.