Don't do hasattr/getattr to get PJSIPUA attributes in Engine

The effect of this is that properties get queried twice

darcs-hash:20090420101618-58103-69fe9a58d2a6d8525704d5e7c3688d74547fd230
Use dir() instead of hasattr() for Engine.__setattr__()

hasattr() also fetches the property from PJSIPUA

darcs-hash:20090420130346-58103-51d9e6b7673902e1f1715d5ce13f153dcdb9d49c