Addresses legacy IE bugs and multiple share_icon conflict

I’ve adjusted the markup slightly to address bugs with older IE
versions, specifically removing the XML namespace attribute and
separating the closing tag from self-closing SVG elements (per
[Stuntbox’s “Making Inline SVG Play Nice in Legacy IE”
post](http://stuntbox.com/blog/2013/06/bulletproof-inline-svg/)).
+
+I’ve also moved the opacity to the CSS on all versions, because I
found that having multiple instances of the share_icons on one page
would conflict with each other (this was probably because of the id
tag, but it also cuts down the size of the [non-cacheable] inline code
a little more).