Search.Regex minimal port

This is a minimal port for Lucene regex-based queries and span queries. The port refers to RegexQuery and SpanRegexQuery only, with their dependencies. The corresponding Java source is quoted in each class remarks. The only renaming was JavaUtilRegexCapabilities which was translated into C# as CSharpRegexCapabilities. No time for adding unit tests and other sources by now, sorry, should I find it I'll make a new post.