dev: improves exception messge of HidingReferenceException and

clarifies some XML documents.
refactor: fixes a typo.
refactor: rectifies a test name.
dev: lets ConstructingMemberAssertion accept only relevant elements.
dev: introduces GuardClauseAssertion.
dev: implements Assertion property.
dev: implements the TestFixture property.
dev: lets GuardClauseAssertionFactory create GuardClauseAssertion.
dev: deletes unnecessary gurad clause in the Create method of

GuardClauseAssertionFactory.
dev: lets GuardClauseAssertion accept only relevant elements.
docs: adds release notes.
dev: inroduces the new constructor

`ConstructingMemberAssertion(ITestFixture)`.
dev: lets `ConstructingMemberAssertionFactory` create

`ConstructingMemberAssertion` with `ITestFixture`.
dev: deletes the `Verify(Assemlby)` method in

`HidingReferenceAssertion` and `RestrictingReferenceAssertion`.