Added Conditional Put

Hello,

I've just started using AmazonSDB and I'm very happy with the project. I needed to be able to do conditional puts in one of my projects, so I have updated my fork of your project to create a new method called putItemConditional with the associated support code. I've tested the code in my own project. I didn't really do much else. 

If you want to add this to your branch, feel free to use it. I don't really need any attribution or anything if you want to use it, but you can if you like. Github username: RealNerd, Name: Blake Schwendiman, Email: blake.schwendiman@gmail.com. Like I said, use it if you like. Ignore it if you want.