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

There are two possible hidden values, `hidden` and `veryHidden`, but I thought it's probably simpler to only expose whether it is hidden or not. I'll be happy to make any changes!