add new method to clean log msg for csv output

Solving [issue 28 - escape before writing for csv](https://github.com/ligershark/AzureJobs/issues/28)
I took the suggested method and refactored a bit, and called this new method from `Logger.Write()`.
I validated with a few [quick tests](https://dotnetfiddle.net/pEBVWp)

![capture](https://cloud.githubusercontent.com/assets/930731/3635254/51ac744a-0f67-11e4-9449-6bd114bf9680.PNG)