Jetty detection

[Dain Sundstrom]

I added code to switch from the OpenEJB http server to Jetty when the  
Jetty classes are available.  You can also explicitly state which  
http server impl will be chosen by setting the 'impl' property in the  
HttpEjbServer properties.

Let me know if you have any problems.  Also, David if you have time  
can you review my changes?  I'm not sure it is how you wanted this to  
work (I just hacked something quickly).

-dain