Tuple Assignment Issue in kml.py Fixed

In fastkml/kml.py on line 451, part of a tuple was being assigned, and this broke on python 2.7.1. The specific code change was:

Before:

```
self._snippet = ('',0)
self._snippet[0] = snippet.text
if snippet.get('maxLines'):
    self._snippet[1] = int(snippet.get('maxLines'))
```

After:

```
self._snippet = (snippet.text,)
if snippet.get('maxLines'):
    self._snippet += (int(snippet.get('maxLines')),)
else:
    self._snippet += (0,)
```