Added size constraints and enhanced save/load functionality

[wjk]

Here is the pull request you asked for earlier. It contains the following changes:

• Added the splitterPositions Objective-C property
• Added the JWSplitViewDelegate protocol
• Added the JWSplitViewDidResizeNotification
• Split JWSplitView.h and JWSplitView.m out into a separate framework. I did this because I happen to like embedded frameworks; edit to taste.
• Updated the README file; you might or might not want to merge this.

[jwilling]

Fantastic, thanks for this! One thing. It seems that the constraining methods don't work on split views other than the first one. This might be my implementation's fault, or it could be that a case wasn't covered in this new code, but in any case it does not seem to be functioning correctly.

If you don't have time to look into it, let me know so I can merge this in and debug it a bit myself, otherwise I'll wait to merge until further notice from you.

[wjk]

I never covered that particular case, as I only used split views with one splitter and two subviews. I'll try to find time to look into this; I would be appreciative if you did as well. Thanks!

[jwilling]

No rush at all. I'm extremely busy at the moment so I can't look at it either until things slow down over here.