Ability to disable SSL session verification

@ncr - this is @pvdb coming to you from our team account here on github.com

The changes in this pull request add the ability to disable SSL session verification in `Rack::Proxy` in case you're targeting e.g. a test server with self-signed SSL certs.

It boils down to doing:

```ruby
http.verify_mode = OpenSSL::SSL::VERIFY_NONE if http.use_ssl?
```

... in all the right places!

Closes ncr/rack-proxy#16