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)

Below is the corresponding encryption 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.