Added size constraints and enhanced save/load functionality

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.