update qrcode.clear() method

[aasdkl]

update qrcode.clear() method and it can be used when useSVG=false add the useSVG in options and update the README.md

[shoaly]

@steplegionnare i encounter the same isssue, trid clear the src attribute on img tag, worked on android, however failed on iphone. the workout for me is clear all inner html of the qrcode container. for example: generate: qrcode = new QRCode($('#qrcode')[0]) clear: $('#qrcode').html('');

[NightSpirit152]

I think clearing inner HTML manually is not proper thing to do) .clear() method needs to be modified to work everywhere)