added additional IE10 Browser test

added additional property to Prototype.Browser to sniff IE10 so that the 
user agent is sniffed once instead of multiple times if IE10 becomes a problem
Added check on already done browser sniff

Removed try/catch that didn't really work for IE10 and added if() check for IE10 property set at top of js code
Merge remote-tracking branch 'origin/IE10-Attr-Test'