Introducing REQUIRE macro

Introducing additional functionality to allow client code to stop a unit test when an assert fails.

The following macro has been added:

	REQUIRE

An example of when these type of checks are useful:

    std::vector<int> v = foo();

    REQUIRE(CHECK_EQUAL(3, v.size())); // test stops here on a fail
                                       // so we don't segfault looking at
                                       // v[0] below.
    CHECK_EQUAL(1, v[0]);
    CHECK_EQUAL(2, v[1]);
    CHECK_EQUAL(3, v[2]);

Multiple checks are supported as follows:

    REQUIRE({
        CHECK_EQUAL(1, 2);
        CHECK(true);
    });

In the multiple check scenario, all the checks in the REQUIRE block will
be run. After which, if any failures were reported, the TEST case will
be stopped.

When UNITTEST_NO_EXCEPTIONS is defined, REQUIRE is a noop.