Three small changes

1. Static Library build target. Also updates the Demo app to use a static library instead of adding the files to the project directly. This shouldn't affect existing uses (cocoapod or otherwise), since the file locations weren't changed.

2. Use string constants instead of their values. This one's likely just for correctness, but, make a change to use NSKeyValueChangeNewKey instead of assuming that @"new" will be the key name.

3. Don't send UIControlEventValueChanged actions until the animation finishes. This fixes a problem that came up during testing where data would come in before the animation finished, leading to weird animation timings when hiding a refresh control.