Don't lost custom indexedColors when edit exist file
Refuse all the operations on row 0 or column 0

As row and column all start from 1 instead of 0
Add assert for row==0 and col==0

As some users consider that (0, 0) is the first valid cell, so we give
a assert failure in debug mode for such cases.
Make write string slightly fast
Code refactor, make use of benchmarks
Make Format() constructed as fast as possible.

QMetaType::type("XlsxColor") is not fast.
Issue #32: Merge branch 'master' of github.com:EddoH/QtXlsxWriter into EddoH

Conflicts:
	src/xlsx/xlsxworksheet.cpp
	src/xlsx/xlsxworksheet_p.h
Fix examples compile error introduced by API change
Code refactoring: add new constructor CellRange(const char *)
Revert hello/main.cpp
Fix some documentation error.