new version v5.2.1 of Html2Pdf

• add cert tag - thanks to @LittleBigFox
• add balloon example - issue #385
• change the name of all the examples from exemple to example
• fix tag parser - css style corruption
• fix issue on relative margin calculation
• fix issue on border radius calculation
• fix issue on page background image width - issue #394
• fix issue on qrcode value with multi lines - issue #390
• fix issue on colspan usage when all the columns don't really exist - issue #360
• fix documentation - thanks to @noxlux and @tpohchai