Some cleaning up

Playing about with the library, thought I'd throw in a few contributions.

a) With the access token, I have added encapsulation for ``accessToken`` from `BufferKit/Client/BFKClient.m` to catch empty or invalid accessToken inputs. You'll probably already know that it supports both the dot notation and bracket notation.

b) Improved the README to have instructions for getting the demo working, given that it does not work under the *.xcodeproj format (or at least to my understanding) and a *.xcworkspace format needs to be generated. The updates reflect this and hopefully should get more people contributing.

Let me know if there's any changes you would like me to make. I'll try make further contributions and have a play on implementing the authentication system sometime soon.