Store credentials per instance to allow multiple clients

[iliran11]

solves #6

• base/base.ts has changed its name and is now /base/transport.ts to better reflect this file is about transporting data to twitter API.
• The methods of base/base.ts are exactly the same in the new file - I was just moving around the functions inside the class.
• Credentials were store as private constants of a file /base/base.ts. Now the new module /base/transport.ts export a class which can be instantiated.
• the credentials are encapsulated inside each Transport instance
• the main class TwitterClient creates a new instance called transport and passes it to all of the clients.
• Each client now requires Transport instance and calling class methods of this instance.

thanks, @LaptopCouch

[iliran11]

Fixed the requests @Silind