Migrates the StringFluentAssertionExtensions class to the new assertion runner pattern.

Refactors and introduces new methods to generate exception messages (string) within the EqualityHelper and IsInstanceHelper classes.
Migrates the StructFluentAssertionExtensions class to the new assertion runner pattern.

Adds more unit test related to the error messages.
Migrates the TimeSpanFluentAssertionExtensions class to the new assertion runner pattern.

Adds more unit test related to the error messages.
Migrates the EnumerableFluentAssertionExtensions class to the new assertion runner pattern.

Adds more unit test related to the error messages.
Migrates the NumberFluentAssertionExtensions class to the new assertion runner pattern.

Adds more unit test related to the error messages.
At last, gets rid of the Value property on the IFluentAssertion and IStructFluentAssertion interfaces.
Replaces all the "which is not expected" by "which was not expected".
Updates the HowToAddANewAssertion documentation (presenting the new assertion runner approach)

Updates the backlog.
Fixes a hard-coded exception message.