CS1520 code issue and auto-fix.

This adds code issues based on actual compiler errors.
I added CS1520MethodMustHaveAReturnTypeIssue.

Previously, issue providers wouldn't even be run for documents with this kind of errors. As such, this will require changes to MonoDevelop (I will send the pull request soon).

First of all, this lets the NRefactory context know about compiler errors.
Additionally, errors now have a field indicating the error code (a string such as `"CS1234"` or `null` for no code).

The issue will detect cases like this:

```csharp
public class Class1
{
   public NotTheClassName()
   {
   }
}
```

It will then suggest two fixes: Make it a void method, or make it a constructor.

Let me know if there was a better way of doing this.