use SharpZip 0.86 for .NET 4 profile
fix NPOI.csproj
update release notes
update readme.txt
update Release note for 2.1.3.1
remove unnecessary file
update assembly version to 2.1.0.0 prepare for the release
2.0.6 RC1
fix some bugs a. XSSFSheet.ForceFormulaRecalculation = false doesn't work b. implement IsActive c. fix a bug in XSSFSheet.TopRow