changed MEF search folder resolving

I were trying to figure out why my loader wasn't attached to CqlSharp. This is what I've got when attached VS debugger to my IIS web site, that uses CqlSharp. 

![cqlsharp-loader](https://cloud.githubusercontent.com/assets/4157587/3625759/ffb51bba-0e71-11e4-8324-e9acea6825ad.png)

As you can see `Assembly.GetExecutingAssembly().Location` points to temporary ASP.NET folder that contains only CqlSharp.dll, nothing more. MEF cannot load any imports in this case ;)
AppDomain.CurrentDomain contains right paths in `BaseDirectory` and `RelativeSearchPath` properties. In case of .exe app the `RelativeSearchPath ` property is null.