Implementation of dictionary and query suggestion indexers

Hey Mike,

This includes the dictionary and query suggestion indexer implementations, so you can ignore David's previous pull. We've added an indexSuggestions method in SearchEngineUtil to enable the triggering from the control panel, in a similar fashion as reindexing dictionaries, but I had to add an empty method in  LuceneSpellCheckIndexWriter. The CustomEntries interface is designed to contain everything about general scopes. This is necessary for QuerySuggester in order to create the filterQueries.