Add search result context in reports - WIP

- Add extracting context & result positions to a more useful class
- Add position-aware stream reader
- Add IDocumentBuilder.AddText
- Add multiple tests for PositionTestPair and PositionTestPairGroup
- Adjust existing tests so that they pass with recent changes
Add automatic NuGet package downloading on build
Fix broken assembly references, insert placeholder PDFDocumentBuilder.AddText
Fix PDFDocumentBuilder.Save and its test
Add Begin/EndContextBlock, rename AddText to AddContextText in IDocumentBuilder