Threadsafe client

This pull request provides the ability to make requests on behalf of multiple trello users from a single application. In doing so, it addresses issue #36. 

Now all API requests go through a configured client, which may be either through global configuration (as before), or through individually instantiated clients. The additions to the README explain how applications using the gem can take advantage. Of course, those who don't need multiple users can continue making requests as they have been.