Remove test fixture builders and incorporate into attributes

This is the next stage of removing / replacing addins in NUnit 3.0.

This pull request covers 
* Elimination of standalone builders for SetUpFixture and TestFixture
* Introduction of IFixtureBuilder interface for use by attributes
* Putting the logic of building the fixtures into SetUpFixtureAttribute and TestFixtureAttribute

SetUpFixtureBuilder is gone completely. NunitTestFixtureBuilder is split into an upper and lower part: DefaultSuiteBuilder and NUnitTestFixtureBuilder. Control logic for building is now...

   TestFixtureBuilder (static) => DefaultSuiteBuilder => SetUpFixtureAttribute
                  "                         =>            "                   => TestFixtureAttribute => NUnitTestFixtureBuilder

NUnitTestFixtureBuilder is also used directly by DefaultSuiteBuilder when no attribute is present.

Of all the suite/fixture builders, only LegacySuiteBuilder remains relatively unchanged. There's no point in changing this if we aren't going to support Legacy Suites (with SuiteAttribute) which is to be decided.

At a certain point in implementing this, working bottom up no longer seemed best, which is why I have moved up to the suites. I expect to do the test case builders next.