add accessibility message if mathjax is detected

use ie conditional html tags to detect for compatible ie versions

lms/static/coffee/src/mathjax_accessible.coffee

@@ -0,0 +1,16 @@
+$ ->
+	isMPInstalled = (boolean) ->
+	# check if MathPlayer is installed
+	# (from http://www.dessci.com/en/products/mathplayer/check.htm)
+		try
+			oMP = new ActiveXObject("MathPlayer.Factory.1")
+			true
+		catch e
+			false
+
+	# detect if there is mathjax on the page
+	# if not, set 'aria-hidden' to 'true'
+	if MathJax? and not isMPInstalled()
+		$("#mathjax-accessibility-message").attr("aria-hidden", "false")
+	else
+		$("#mathjax-accessibility-message").attr("aria-hidden", "true")