(on incoming messages)

Illustrated via tests.

Otherwise the index of the element relative to which we place it changes
depending on whether the element being positioned is placed before or after it.

Conflicts:
	builds/converse-no-locales-no-otr.min.js
	builds/converse-no-otr.min.js
	builds/converse.min.js
	builds/converse.website-no-otr.min.js
	builds/converse.website.min.js

* Make sure that the roster container has a fixed height, so that we can better
check whether the roster is overflowing.
* Call update after inserting the roster into the DOM

only shows when the roster is overflowing. updates #212