reduce log level of annoying log entries (that are not really warnings)
read blank cells as null value, empty cells as empty strings
reduce log level
Merge pull request #197 from dennishendriksen/testing

ExcelReader: blank cells = null value, empty cells = empty string
Merge branch 'testing' of github.com:molgenis/molgenis
skip empty rows during importing
Merge branch 'testing' of github.com:dennishendriksen/molgenis