Feature: OfType Operator

[dwcullop]

Description

A new operator OfType that is similar to the LINQ operator with the same name. It converts a changeset of some base type (or interface) to a changeset of a different type by automatically excluding values that can't be cast as the new type. It effectively combines ImmutableFilter and ImmutableTransform into a single operator for maximum efficiency for the case when both operations are trivial.

Bonus Changes

Creates Test-compatible version of the Spy (named TestSpy) that emits output to ITestOutputHelper so that the contents of the Observables can be seen in the test results.