Write conditional formatting styles to worksheet instead of copying to e...

Write conditional formatting styles to worksheet instead of copying to each individual cell. 

See issue #171. This change reduces the size of the generated XML, especially for large ranges, and does not appear to cause any harm. The original behavior looked kind of crazy, to me, and definitely makes Excel2012 unhappy.

Conditional formatting can now apply to a very large range without ridiculously inflating the time it takes to rewrite the XLSX. I have successfully used range sizes up to the maximum that Excel 2010 supports.