Don't emit some empty xml elements.

Fixes Excel (2007) corruption warning.
Fixes claims of MS Excell 2010 for generated test.xlsx.
TODO fix created and modified tags in core.xml
some code deduplication and cleanup
Speedup collect of shared strings
Some renames and cleanup
test.xlsx shouldn't be in this repository
creator should not appear in title and subject
Fixed core.xml

Namespaces rearranged properly, removed unnecessary version
element and added lastModifiedBy (MS Office sets it by default)
Closes #14