Support for efficient deep paging in lucene.net

[nzdev]

Support for efficient deep paging in lucene.net.
Uses SearchAfter instead of Search when the SearchAfterOptions are available.
This allows lucene to efficiently skip documents before the last document from the previous page fetched for a search.

Steps:

1. Build and execute your query as normal.
2. Cast the ISearchResults from IQueryExecutor.Execute to ILuceneSearchResults
3. Store ILuceneSearchResults.SearchAfter (SearchAfterOptions) for the next page. It may be worth serializing this class and cryptographically hashing it to prevent tampering in a web application so that it can be made available to the next request for the next page.
4. Create the same query as the previous request.
5. When calling IQueryExecutor.Execute. Pass in new LuceneQueryOptions(skip,take, SearchAfterOptions); Skip will be ignored, the next take documents will be retrieved after the SearchAfterOptions document.
6. Repeat Steps 2-5 for each page.

Added documentation.
Added tests.

[Shazwazza]

Nice! and looks like it doesn't introduce breaking changes which is a winner. Have added some comments.