Introduces the 'Not' operator and implements it for the <string>Contains(string[] values).
Adds more unit tests related to the Not operator.
Disables the SA1202 StyleCop rule so that every 2 private methods  (one for negate, the other for the non-negated version) related to every public extension method for assertions may be located just after.
Extracts 2 private methods for handling the negated and non-negated <string>Contains() assertion.
Makes the <string>StartsWith method negatable.

Now, the ChainableFluentAssertion creates a new instance of the same assertion type in order to prevent from spreading the Negatable property value from an assertion to the next one. Thus, modify the IFluentAssertionBase to do that.
Renames few methods and interfaces around the chaining process of assertions (which are now 'forked').

IFluentAssertionBase is now renamed into IForkableFluentAssertion.
Improves the documentation level of the new IForkableFluentAssertion.ForkInstance() method.