Makefile for ubuntu and user/passcode in connect frame

Hi!

I'm using STOMP for some project (with HornetQ) at the moment and noticed that your library matched my needs quite well. However, two things were missing for me: out of the box compilation on ubuntu and username/password in the connect frame. 
The first was solved by adding -lpthread in the make-globals. The only thing im not sure about though is whether this breaks on non-ubuntu platforms or not... If so, then the makefile needs some further tweaking.
For the second point I added a new start(user, passcode) function that includes this information in the header as long as user is not an empty string (the STOMP 1.1 specs only say that the client MAY include user and passcode in the header, not clear whether it is allowed to send only one of the two). The old start() function is still there and behaves in the same way as before.
Anyway, the changes are simple, if you feel these changes can be useful for other people, please pull them in. 

Cheers,

Remco