Crash on start with Python 2.7.

On Python 2.7 there's a problem in yowsup-library, that can lead to a crash on start, because of unicode decoding error. This needs more investigation, but can be worked around for now.

To fix the issue edit yowsup/structs/protocoltreenode.py and change line 63 
from

``` python
out += "%s" % self.data
```

to

``` python
try:
  out += "%s" % self.data
except UnicodeDecodeError:
  pass
```
