Adds the reference to Rui within the metadata.
Adds the LinkForNextCheck property to the IRunnableCheck interface + related tests.
Refactors the extensibility part; few renaming on the interfaces, methods and properties (e.g.: IRunnableCheck-->IChecker; IRunnableStructCheck-->IStructChecker)

Adds tests on the extensibility for struct to end the support of IStructCheck linkage.
Fixes the HowToAddANewAssertion.md + refactors the extensibility.
Updates the version number.
Updates the release note.
Improves IChecker documentation.
Improves the IChecker documentation again;
Cleans dead using
Gets rid of a useless [EditorBrowsable] attribute within the IWithValue interface.
Refers the Codedistillers "NFluent Extensions" article within the markdown documentation.
Adds a link on Rui name within the markdown doc.
Merge branch 'extensibility'

Conflicts:
	ReleaseNoteContentToBeInsertedWithinNuspecFile.txt