Allow comparing API versions
Allow including multiple API versions in a single controller descriptor
We're going to represent a unknown API version as 'UndefinedApiVersion'
Defining an unknown API. We will also ensure that comparisons with other API versions will also cause unknown API versions to end up low.
Returns now UndefinedApiVersion instead of null, these changes also make the tests pass again
Implemented basic API documentation: at least all api's are returned and the 'latest' api version is shown on the documentation page. Also unversioned API's are returned and shown by the default API explorer.
Use 'this' qualifier as per convention
Add 'VersionedApiExplorer' type which will be the implementation of an API-versioning aware API explorer. For now, it just wraps the default Api Explorer.