Conditional run attributes

Added two attributes that allow tests to be ignored on particular operating systems or runtime frameworks. We need this for the downlevel CI runs.

There is a little bit of code duplication because of how xunit implemented `Theory` and `Fact`...

Example usage:

```
[ConditionalFact(
    ExcludedFrameworks = RuntimeFrameworks.Mono, 
    ExcludedOperatingSystems = OperatingSystems.Windows7And2008R2)]
public void MyTest()
{
    ...
```

@bricelam @Tratcher 
None should be set to 0.
I think you're right, CoreCLR issues just wouldn't compile. We've seen a few implementation bugs, but not many.
What would you name the opposite of Mono? DotNet?