I am doing this with an Arduino with a Smart Things shield. I am using @ogiewon’s amazing ST_Anything library and the device driver and multiplexer app that he wrote. It works great and can save you a ton of money.
Start here:
ST_Anything
If you are new to ST, a lot of what is presented here can be overwhelming, but there are so many devices out there that require this kind of knowledge to properly use them, that this is a good skill to learn if you are serious about ST Home Automation.
Read through the start of the thread, and try to go through the instructions. When you get stuck, post questions there. @ogiewon is very responsive and has helped many people with their projects.
-Todd