performance in jid : optimisation

When profiling our app using sleek, we observed than it was spending a significant amout of time in jid.__setattr__ and jid.__getattr__ : (first number is the position ; the higher it is means more time spond in this method)

````bash
grep -n jid old/stats_tsub.txt | head                                                                                                                                 
16:../sleekxmpp/jid.py.__getattr__:510  28054         0.278681  0.756717  0.000027
37:..ges/sleekxmpp/jid.py.__init__:434  15486         0.145309  3.125407  0.000202
41:../sleekxmpp/jid.py.__setattr__:532  31716         0.126427  0.210283  0.000007
95:.._jid.py.customer_name_from_jid:17  7616          0.055446  0.454063  0.000060
107:../sleekxmpp/jid.py._format_jid:319  9755          0.045821  0.084285  0.000009
109:../sleekxmpp/jid.py.__getattr__:510  4537          0.045728  0.123137  0.000027
140:..kxmpp/jid.py._validate_domain:176  744           0.033182  1.168661  0.001571
180:..ges/sleekxmpp/jid.py.__init__:434  2323          0.022359  0.593739  0.000256
190:../sleekxmpp/jid.py.__setattr__:532  4803          0.019310  0.032180  0.000007
256:..ges/sleekxmpp/jid.py.<lambda>:108  19002         0.011284  0.011284  0.000001
````

This implementation is much more verbose but faster, especially if you are dealing a lot with JIDs
on my box, ./testall.py now takes 45s. It takes 53s in the old implementation (about 15% faster). Our app spends much less time in jid.py :
````bash
grep -n jid stats_tsub.txt | head                                                                                                                                           
50:..ges/sleekxmpp/jid.py.__init__:434  15891         0.102051  2.987120  0.000188
111:../sleekxmpp/jid.py._format_jid:319  9998          0.046407  0.085404  0.000009
112:.._jid.py.customer_name_from_jid:17  7847          0.046369  0.145187  0.000019
136:..kxmpp/jid.py._validate_domain:176  771           0.035105  1.212238  0.001572
206:..ges/sleekxmpp/jid.py.__init__:434  2424          0.016269  0.587972  0.000243
225:..ackages/sleekxmpp/jid.py.full:545  6212          0.013909  0.067937  0.000011
248:..kages/sleekxmpp/jid.py.domain:537  15313         0.012276  0.012276  0.000001
255:..ges/sleekxmpp/jid.py.<lambda>:108  19643         0.011927  0.011927  0.000001
280:..ko_full_jid.py.build_surfer_jid:5  1733          0.009301  0.039370  0.000023
304:.._jid.py.customer_name_from_jid:17  1477          0.007829  0.024348  0.000016
````

* all tests 
* all tests of my app ok
* all acceptance tests of my app ok

Thanks