decrypt.php initial implementation.

Hi Rob,

I've finally made the first version of the decrypt.php through a lot of research and tests.

I am using the CPCryptController for encrypting in iOS side.
(https://github.com/iosptl/ios6ptl/blob/master/ch15/CryptPic/CryptPic/RNCryptManager.m)

Corresponding encrypting codes in iOS to use the decrypt.php.
```objective-c
    // Corresponding iOS encrypt codes example.
	// Use the CPCryptController from https://github.com/iosptl/ios6ptl/blob/master/ch15/CryptPic/CryptPic/RNCryptManager.m
	if (! [[CPCryptController sharedController] encryptData:srcData password:kRNCryptorPasswd error:&error] ) {
		NSLog(@"Could not encrypt data: %@", error);
	}
	
	// Do Base64 encoding for AES128 encrypted data
	NSString *saltBase64 = [[CPCryptController sharedController].salt base64EncodedString];
	NSString *HMACSaltBase64 = [[CPCryptController sharedController].HMACSalt base64EncodedString];
	NSString *ivBase64 = [[CPCryptController sharedController].iv base64EncodedString];
	NSString *encryptedDataBase64 = [[CPCryptController sharedController].encryptedData base64EncodedString];
	NSString *HMACBase64 = [[CPCryptController sharedController].HMAC base64EncodedString];
	
	// Build the final format
	NSString *encryptedDataBase64Final = [NSString stringWithFormat:@"%@%@%@%@%@",saltBase64, HMACSaltBase64, ivBase64, encryptedDataBase64, HMACBase64];
	DebugLog(@"encryptedDataBase64Final: %@",encryptedDataBase64Final );
	
	//If no error we send the post, voila!
	if (!error) {
		
		NSMutableDictionary *params = [[NSMutableDictionary alloc] init];
		
		// Post enctypted
		[params setObject:encryptedDataBase64Final forKey:@"JASON_Enc_Base64"];
		
		// Do Post!
		[[RKClient sharedClient] post:postResourcePath params:params delegate:self];
		
	}
```

I hope this help.
Thanks.