RELEASE_0.9.1.1

RELEASE_0.9.1 of XlsxWriter, plus feature and performance improvements for Tervela:

1. Added support for disabling the filter button on specified autofilter columns
2. Added support for specifying the number of rows to keep in memory before writing to disk while in constant_memory mode
3. Added support for specifying columns that should use shared strings even while in constant_memory mode
4. Sped up output of constant_memory rows when the difference between col min and max is large due to formatting (like setting the background color for all cells)
5. Sped up the output of formats that match other formats by keeping track of the matching format's index