Add validate_cert member to tornado AsyncClient, to enable SSL sentry qu...

Fixes https://github.com/getsentry/raven-python/issues/402

Currently it's not possible to pass the *validate_cert* option to the sentry reporter. When using raven with a non-validatable server via HTTPS transport, the reporting doesn't work and a warning message is printed (see https://github.com/getsentry/raven-python/issues/402). 

This pull request allows a statement like this:
```py
sentry_client = AsyncSentryClient(
            'https://foo:bar@foobarcom/2'
)
#That's new!.
self.sentry_client.validate_cert = False
```

The option is passed on to *AsyncHTTPClient.fetch()*.

By default, *validate_cert* is True (therefore the default behaviour is not modified by this patch).

Merging would be highly appreciated!