Create new super-type for FactAttribute that is the basis of extensibility

[bradwilson]

In v2, FactAttribute is the base attribute that is used to identify a test method in the default test framework implementation (the way the majority of xUnit.net users use it). As an extensibility point, you can derive from FactAttribute and implement changed behavior; an obvious example is TheoryAttribute, which allows the user to write parameterized (data-driven) tests.

There are implied implementations of behavior that currently reside in FactAttribute: namely, that you can get/set the display name, skip reason, and timeout for a given test. From an extensibility perspective, it would be nice to lower these requirements so that it opens the door for more flexible definitions of test methods, without requiring a heavier-weight implementation of the test framework infrastructure.