Allow the clients to decide if validation of SSL certificates should be skipped

Instead of just assuming that in DEBUG root certificates should not be evaluated allow the callers to decide.
In order to not break existing clients, in DEBUG evaluation still is off, but in release builds you can set the allowUntrustedSSLCertificates flag.
This particular style of putting an if condition in an `#if` doesn't jive well our iOS codestyle. I'd write it more like this:

```
BOOL allowUntrustedSSLCertificates = self.allowUntrustedSSLCertificates;

#if DEBUG
allowUntrustedSSLCertificates = YES;
#endif

if (allowUntrustedSSLCertificates) {
    [SSLOptions setValue:@(NO) forKey:(__bridge id)kCFStreamSSLValidatesCertificateChain];
    SRFastLog(@"Allowing connection to any SSL certificate");
}
```