Many changes

(moved clj-excel to vinzi-namespace to take it along during logging)
Small fix (typo)
Better formatting of the poi-excel-interface.

The base-workbook now only has a single sheet.
Now setting types and formats correctly for the strings, dates, etc..
Original versiond did not set types, which might result in issues
when reading back data (although things look fine in excel.)
improved vinzi.clj-excel + small changes

vinzi.clj-excel now uses a workbook that keeps track of its styles
and current sheet.
Next step would be to encapsulate functionality in an object/hashmap/closure
too (for a cleaner interface).
Small updates/additions.