Allow to set SSL method to use when connecting to https sites

some broken sites fails to scrape if we let SSL method to be negotiated between scrapy client and site server, but they work if method can be explicitly set to TLSv1

```
$ scrapy fetch https://208.86.169.96/
...
2012-11-07 17:05:57-0200 [default] ERROR: Error downloading <GET https://208.86.169.96/>: [('SSL routines', 'SSL23_READ', 'ssl handshake failure')]
...
```

This patch allows to override the ssl method by passing the special meta key `ssl_method`