Harden KVO by using selectors instead of strings.

This way, refactoring is less dangerous. Also properly use NULL for context.
Add note and todo.
v0.1.2
Update podspec source attribute.
Fixup docs. Other minor cleanup.
Fix minor syntax error.
Remove unneeded KVO. Also use context.
Improve performance by hiding off-screen children when possible.
v0.1.3
Fixup KVO (04e74c5).