Make assertions into a 1st class feature in the test framework

We recently discovered that the assertions provided by NUnit do not provide any support for primitive types and in those cases will default to `object` which causes boxing/unboxing when doing assertions. This can be a huge drain on performance when this operation is done in a tight loop.

It is clear that end users will likely run into similar issues with testing that we did. While we have addressed the problem, we did so in 2 assertions types that are both internal. We need to create a public API for assertions that end users of the test framework can use.

- Missing overloads need to be added to support all of the operations that NUnit does
- Custom asserts that we created need to be expanded to include all primitive types
- A logical way to make these asserts "native" in `LuceneTestCase` and all of its subclasses needs to be devised

In Lucene, `LuceneTestCase` simply subclassed `Assert`. However, due to a difference in naming conventions between NUnit and JUnit, this would seem odd. For example, instead of `AssertEquals`, the NUnit team named their method `AreEqual`. While this isn't such a problem, it would be inconsistent with all of the other custom assertions in the test framework (i.e. `AssertAnalyzesTo`).

In addition, API documentation should be provided for each of the assertions.