Added support for client SSL certificate authentication.

Usage sample snippets below:

```objective-c
NSURL *url = [NSURL URLWithString:@"wss://ssltest.domain.com/1/ws/"];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSString *path = [[NSBundle mainBundle] pathForResource:@"certificates" ofType:@"p12"];
NSData *pkcs12data = [[NSData alloc] initWithContentsOfFile:path];
SRWebSocket *_webSocket = [[SRWebSocket alloc] initWithURLRequest:request andClientCertData:pkcs12data andClientCertCipher:@"cipher"];
```

This code assumes that:

* PKCS #12 file is named "certificates.p12" in app bundle
* The key passphrase is "cipher"