Using inline strings (which currently don't display in LibreOffice or OpenOffice).
Streaming solution works in LibreOffice and OpenOffice.
Eliminated optional attribute.
Renamed varilables to make intention explicit.
Merge branch 'inline-strings'
Bumped version.
Adjusted compression level to improve performance.
Using inline strings instead of 'str' (which are formulas: http://stackoverflow.com/questions/6468783/what-is-the-difference-between-cellvalues-inlinestring-and-cellvalues-string-in)