V4 Similarity support

[nzdev]

What?

This pull request adds support for setting the Similarity used for querying a Lucene Index.

Why?
This can improve the scoring of the documents in the index for more relevant results.

Example:

var searcher = (BaseLuceneSearcher)indexer.Searcher;
var query = searcher.CreateQuery("cOntent",
                    BooleanOperation.And,
                    searcher.LuceneAnalyzer,
                    searchOptions: new LuceneSearchOptions
                    {
                        Similarity = LuceneSearchOptionsSimilarities.BM25
                    }).All();
 var results = query.Execute();

How?

• Added SimilarityDefinitionCollection to IndexOptions to set up available similarity definitions
• Added IndexSimilaritiesFactory to IndexOptions to set up available similarity types
• Added LuceneSearchOptions.SimilarityName property to set the Similarity on a query.
• Added SimilarityDefinitionCollection.AddExamineLuceneSimilarities() with LuceneSearchOptionsSimilarities with a set of preconfigured defaults.
• Set LuceneSearchOptionsSimilarities.ExamineDefault to the existing default similarity (Lucene.Net.Similarities.DefaultSimilarity)
• Added recommendation to change LuceneSearchOptionsSimilarities.ExamineDefault to BM25Similarity in Examine V5
• Added DictionaryPerFieldSimilarityWrapper as a simple way of setting up per field Similarities.
• Added documentation
• Added tests

Similarity types

These are the default similarity types provided with Examine.

Similarity Name	Description
Examine.Default	Default Similarity for Examine Lucene. ( V3/V4 Lucene.Classic), (V5 Lucene.BM25)
Lucene.Classic	Classic Similarity for Lucene
Lucene.BM25	BM25Similarity with default parameters for Lucene
Lucene.LMDirichlet	LMDirichletSimilarity with default parameters for Lucene
Lucene.LMJelinekMercerTitle	LMJelinekMercerSimilarity with parameter 0.1f which is suitable for title searches
Lucene.LMJelinekMercerLongText	LMJelinekMercerSimilarity with parameter 0.7f which is suitable for long text searches.

[nzdev]

While this does work fine. I'd like to go further with this.