Faraday connection

What
====

The internals of the linkedin gem are in need of some refactoring. This changes the internals to rely on Faraday and FaradayMiddleware to reduce some of the cruft. 

The tests pass but I'm weary as to whether this is good enough yet or not.

One thing that I added was the ability to add middleware to linkedin so that for instance you could do something like this
```ruby
LinkedIn.middleware << FaradayMiddleware::Caching
```

@hundredwatt @cmhobbs would love some second eyes on this.