Allow dr to run on machines with only .net 4.0+ installed.

I added a &lt;startup&gt; element to the app.config for newer machines without 2.0/3.5 installed. 

According to [msdn](http://msdn.microsoft.com/en-us/library/jj152935.aspx), the supportedruntime elements added in this order will first try to use .net 2.0/3.5 if installed, and fallback gracefully into .net 4.0/4.5 if that is installed.

I had to do this to get dr running, and it does work as expected.

I also added the All files filter to the save dialog, just in case the format to save is not .txt/.rtf.