Better name completion

Hi there,

I made a change to the name completion so it behaves more like other jabber clients I have used. The changes are:

* Case insensitive test against the name
* Completion at the start of a line will also append a colon and space e.g. "Nickname: "
* Completion elsewhere in a line will only append a space e.g. "Nickname "

This is how tab completion would be expected to work in most IRC clients and this standard seems to have been carried into xmpp chat rooms.

Cheers,
Iain