Encapsulates the runnableCheck and its Value into the CheckRunner.
Introduces the NFluent.Extensibility namespace, with new types and some old one that are now public (and not internal anymore)

Improves the way checks are implemented thanks to the brand new ExtensibilityHelper<T> that allow to extract both the value and the runner from the fluent check. As of today, this new way is only implemented for the BooleanCheckExtensions methods and the CharCheckExtensions ones.
Migrates the ComparableCheckExtensions class to the new extensibility model.
Migrates the DateTimeCheckExtensions class to the new extensibility model.
Migrates the DictionaryTimeCheckExtensions class to the new extensibility model.
Adds comments about the crappy cast which is now encapsulated within the ExtensibilityHelper type.
Refactors the extensibility mechanism, by allowing a single call to the ExtensibilityHelper.
Gets rid of the ExtensibilityHelper.ExtractValue() method, since the value is now exposed by the runnableCheck.
Fixes the documentation of the ExtensibilityHelper class.
Renames IRunnableCheck to ICheckForExtensibilty, IRunnableStructCheck to IStructCheckForExtensibility, ICheckRunner to IRunnableCheck, etc.
Updates the release note.
Updates the version number to 0.11