Fixing IE browser sniffing (was broken in IE9)

IE9 browser sniffing for lib.js' isIE method was broken and returns false when it should return true.

I don't know why isIE was doing boolean logic string coercion gymnastics rather than leveraging the user agent.  Is is there a good reason?  

This pull request leverages the user agent, which works more reliably in my IE browser tests.