Issues with adding cells with a formula, and saving a row with no cells
Fixing tests
Issue #93: adding handling of thumbnails; adding png content-type
Version bump to 2.4.2
rdoc update
Regenerate gemspec for version 2.4.2
Merge branch 'master' of github.com:weshatheleopard/rubyXL into performance_enhancers

Conflicts:
	lib/rubyXL/worksheet.rb