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 
Why do we consider Win7 & 2008R2 separate OSs? I'm not aware of any meaningful distinction between the two. Win7 vs Win7Sp1 has more meaning. Consider just using Windows7And2008R2 as a single flag.