Warn about IE emulation modes in docs+examples

This script, which I've pretty exhaustively tested manually using Sauce, alerts the user about the perils of trying to use IE's emulation modes to test older versions of IE.
These emulation modes can behave differently from actual IE (see #13177, http://stackoverflow.com/questions/5612170/web-testing-for-ie-how-accurate-is-ietester), which can lead users to file Bootstrap bugs about supposed IE8 problems which don't occur when tested properly using actual IE8.
This should hopefully reduce the amount of time wasted chasing down false IE8 bug reports.
/cc @twbs/team for review