Implement `async` flag in options

[lyndsysimon]

When instantiating a new Gab object, passing the option async: false should allow the library to function without returning promises.

var Gab = require('gab.ai').Gab;
var api_client = new Gab({
    authToken: '<your JWT here>',
    async: false
});

api_client.getUser('LyndsySimon');
// { ... }

api_client.getUser('UserThatDoesNotExist');
// ValueError: User does not exist.