Fixed eval in the _to_python method

_to_python method is unsafe:

``` python
    from pysolr import Solr
    solr = Solr('http://localhost:8983/solr/collection1')
    solr.add([{'id': 'unsafe', 'name': 'open("/etc/passwd").readlines()'}])
    res = solr.search('id:unsafe')
    print(solr._to_python(res.docs[0]['name']))
```