updated licence tag
import unit tests for workbook
fixed small bugs in Workbook
added unit tests for worksheet
refactored cell method "coordinate" into "get_coordinate"
added zip lib wrapper
added stub for writer
added a genuine empty workbook to use in tests
added package paths to shared zip class
added read methods for document properties
refactored properties reader
added licence and credits
moved office open xml format data to a specific module
added module for handling dates and times
finished tests for document properties reading
added writer for properties
changed the start number of sheet name (1 instead of 0)
added [content types].xml writer
added writer for app.xml
renamed reader for core props