Fix copy/paste in IE

This fixes a problem with the rrunmode addon in which copy and paste in IE results in newlines being stripped.

A related problem with [google-code-prettify](https://code.google.com/p/google-code-prettify/) is discussed [here](http://stackoverflow.com/questions/136443/why-doesnt-ie7-copy-precode-blocks-to-the-clipboard-correctly).

The solution in this pull request is to change to using newlines instead of `<br/>`.

Plus there is some special handling for IE8 and earlier, inspired by [google-code-prettify](https://code.google.com/p/google-code-prettify/source/browse/trunk/src/prettify.js#1203)