Document boost support

Added support for document boost.

Included examples for [Fluid API](https://github.com/tamasflamich/Lucene.Net.Linq/blob/master/source/Lucene.Net.Linq.Tests/Samples/FluentConfiguration.cs) and [attributes](https://github.com/tamasflamich/Lucene.Net.Linq/blob/master/source/Lucene.Net.Linq.Tests/Samples/AttributeConfiguration.cs).