update NPOI version

Update .gitignore

ignore idea folder

fix compilation issue for ExcelStyleDateFormatter.cs

Delete NPOI.2.5.1.nupkg

fix nullexception in IsMergedCell

fix nissl-lab#275

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