Provide a MUC method to create *or* join a room

MulitUserChat.create() will throw an SmackException if the MUC service
does not return a 201 status when entering a room. Some MUC
implementations don't return the 201 status but instead behave like
the room already existed.

If the user doesn't care about the room beeing locked until the
initial configuration has been send, he can now use the new
MutliUserChat.createOrJoin(String) method.

Also remove some duplicate code by creating the private enter() method.

Fixes SMACK-557