Replace templating with Sprintf for cells
Reduce templating and sprintf calls in CellIndex
Remove templating for sheet end
Remove tests for old templates
Merge pull request #6 from sean-duffy/string-performance-improvements

String performance improvements
Use zipWriter.CreateHeader instead of zipWriter.Create
Add level 1 flate compressor, not working
Move RegisterCompressor into NewWorkbookWriter
Introduce constant for new compression method
Use regular deflate for now