fix legacy IE issue with ampersands

Fixes #796

IE version 6-8 had issues with setting innerHTML with & (ampersand characters). This manifested as a bug in Highcharts. See this fiddle http://jsfiddle.net/W7972/3/ for a test case.

See http://jsfiddle.net/hexRz/ for the fixed case.

The fix involves replacing '&' characters (that are not already &amp;amp;) with &amp;amp; before setting it as HTML. This code runs for other standards compliant browsers too with useHTML set to true but is harmless.

PS: I didn't stage the assembled files in the commit because they were showing diffs that aren't mine.