Implement "Spec Way" expectation syntax

[prowwid]

"Spec Way" implementation requires switching form "$this->tester" notation to using "I" operator implemented by static class so that spec syntax will be similar to:

        I::describe('income tax calculations');

        I::expect('for income less that 50 000 calculator use 10% tax rule', function () use ($service) {
            I::lookAt('first level income tax')
                ->seeNumber($service->calculateTax())
                ->isNotEmpty()
                ->isEqualTo(self::EXPECTED_TAX_FOR_FIRST_LEVEL_TAX_RULE);
        });

        I::expect('for income between 50 000 and 100 000 calculator use 12% tax rule', function () use ($service) {
            I::lookAt('second level income tax')
                ->seeNumber($service->calculateTax())
                ->isNotEmpty()
                ->isEqualTo(self::EXPECTED_TAX_FOR_SECOND_LEVEL_TAX_RULE);
        });

        I::expect('for income more than 100 000 calculator use 20% tax rule', function () use ($service) {
            I::lookAt('second level income tax')
                ->seeNumber($service->calculateTax())
                ->isNotEmpty()
                ->isEqualTo(self::EXPECTED_TAX_FOR_THIRD_LEVEL_TAX_RULE);
        });