Add CsvClassAttribute

Using this wonderful lib, I've got an idea of marking the class properties not only with maps, but with attributes (see example in CsvClassMappingAutoMapTests.cs). I think, it will make lib even easier to use - you can map different titles even without using Mapping class.

The logic of attribute is same as abstract keyword in C#:
- if class is marked as CsvClass, only properties with CsvProperty attribute are mapped
- if class has a CsvProperty attribute, it's counted as if it was a CsvClass

I've wrote UnitTest for it, but I couldn't run it :( (I don't know why, VS simply says that I have no unit tests in app).

I think, that attribute can be extended even more (supporting ignore and other commands)