Fixed possible bug in process for getting  be subscribed to push-notification cha...

...nnels in first time launch of the app.
At first time you launch the app, the first method called in Backbeam.m is:
- (id)initInstance
{
    self = [super init];
    if (self) {
        ....
        self.deviceToken = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
        .....
    }
    return self;
}
And in this first time, the result is always nil, so we have self.deviceToken as nil.
Later is called this method:
- (void)persistDeviceToken:(NSData*)data {
    NSString* base64 = [data base64EncodedString];
    NSString* path = [self.basePath stringByAppendingPathComponent:kDeviceTokenPathComponent];
    // TODO: handle error
    [base64 writeToFile:path atomically:YES encoding:NSUTF8StringEncoding error:nil];
}
But the new value is not assigned to self.deviceToken, so whenever you want to suscribe your app to push-notifications chanels with this methods:

+ (void)subscribeToChannels:(NSArray*)channels
                    success:(SuccessBlock)success
                    failure:(FailureBlock)failure;

+ (void)subscribeToChannels:(NSArray*)channels;

+ (void)subscribedChannels:(SuccessArrayBlock)success
                   failure:(FailureBlock)failure;
+ (void)unsubscribeFromAllChannels:(SuccessBlock)success
                           failure:(FailureBlock)failure;

+ (void)unsubscribeFromChannels:(NSArray*)channels
                        success:(SuccessBlock)success
                        failure:(FailureBlock)failure;
+ (void)unsubscribeFromChannels:(NSArray*)channels;

you get always this message from log:
"UnknownDeviceToken"