Name existing S.ieMode to S.legacyIeCompatibilityMode.

We keep S.ieMode as a deprecated value until 3.0.
Include user agent calculation functions in S.
Add helpers for detecting IE10/11.

I know we were all hoping that by IE10 they'd be up to speed with the
rest of the world, but I still remember some support issues, so I'm
adding a detector for it in the event server side detection is needed
for some people. Ditto for 11. Here's hoping these will be the last
versions of IE that ever need such detection.
IE < 6 still counts as IE.

Though we sincerely hope to never see any of these user agents, we
should return the correct value for isIE if they hit our apps.
Comments should reflect reality: update IE compat comments.

We also consider IE 8 a legacy IE.