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

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