Add boolean Hidden field on Sheet

A sheet may be hidden, as specified by the state attribute on the sheet tag. It is good to provide
this information to users of the API. See [spec](http://msdn.microsoft.com/en-us/library/office/documentformat.openxml.spreadsheet.sheetstatevalues.aspx) for more details
Merge pull request #66 from hermanschaaf/hidden-sheets

Add boolean Hidden field on Sheet