Create NUnitLiteFrameworkDriver and modify code to select correct driver

This PR adds changes needed to run NUnitLite tests through the engine and prepares for the future ability to have third-party framework drivers.

The DriverFactory now examines the assembly to determine which of the known frameworks is being used by the test assembly. It creates either an NUnitFrameworkDriver or an NUnitLiteFrameworkDriver accordingly, or gives an error message if the driver is unknown.

Currently, this is hard-coded using a simple switch statement. At a future point, it will be based on a lookup of available drivers and we'll allow third-party drivers to be installed similarly to the way NUnit 2.6 allowed for user-created addins.

This PR can be merged independently of the corresponding framework PR without any problem, but the feature won't actually work until both are merged.