add validation for Worksheet#state and WorkbookView#visibility
Added workbook views collection and workbook view object
Integrate workbook views and alter serialization

This integrates workbook views and sheet state into serialization.

I also noticed that we were populating defined names during
serialization. While it is good to delay this as late as possible
as there is always the chance that some conditional programming
by the consumer adds, and then removes an autofilter, I am
choosing to risk it at this point for cleaner code.
Adds examples for hiding sheets and workbook view

These examples show using hidden and very hidden to manage
worksheet visibility as well as using the active_tab
attribute of a workbook#view in order to specify which
tab should be showing when the sheet is opened.
Merge pull request #250 from randym/bookview_sheetstate

Managing Sheet Visibility and Tab Selection