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 
We could always make this type public if that's the case. This type is meant to address the most commonly occuring condition that our tests run into and isn't meant to provide exhaustive conditionals.