A wall switch that has everything included takes the place of your conventional switch so the process of adding 'smart switches' is as little invasive as possible. If you know how to change a wall switch you can automate your house.
The whole project works around the cheap RF transmitters. All the data is sent encrypted so it won't be easy to someone outside of the home system to get ahold of it. The teensy will play an important part, as its faster and 32bit. With its help, working with cryptography keys will be fast.
Using the electric wiring already present in the box, an AC-DC converter will keep the electronics powered.
A relay controls the system making the outside switch and itself behave as a three-way switch.
The greatest challenge here will be to fit everything inside an electrical box, but as soon as everything works, sourcing smaller components and changing the board layout will help reduce its size.