Sendpulse API initialization

@param user_id
@param secret
@param storage
@param callback
function init(user_id, secret, storage, callback) {
    API_USER_ID = user_id;
    API_SECRET = secret;
    TOKEN_STORAGE = storage;

    if (!callback) {
        callback = function() {}
    }

    if (!fs.existsSync(TOKEN_STORAGE)) {
        mkdirSyncRecursive(TOKEN_STORAGE);
    }

    if (TOKEN_STORAGE.substr(-1) !== '/') {
        TOKEN_STORAGE += '/';
    }

    var hashName = md5(API_USER_ID+'::'+API_SECRET);
    if (fs.existsSync(TOKEN_STORAGE+hashName)) {
        TOKEN = fs.readFileSync(TOKEN_STORAGE+hashName,{encoding:'utf8'});
    }

    if (! TOKEN.length) {
        getToken(callback);
        return;
    }

    callback(TOKEN)
}