Allow for custom Faraday middleware

[Aupajo]

Addresses #29.

Introduces a method called middleware on the Client, that can be used like this:

client = DropboxApi::Client.new

client.middleware.prepend do |connection|
  connection.request LoggingMiddleware
end

client.middleware.append do |connection|
  connection.use InstrumentationMiddleware
end

The order of middleware is important, so prepend will let you set up the connection before DropboxApi modifies it (via an endpoint's builder.build call – usually in an initializer) and append will let you alter the connection after DropboxApi modifies it.

As the adapter must be the last item of middleware, there's one extra method on the middleware object, adapter=:

# Use the Excon gem instead of Net::HTTP to make requests
client.middleware.adapter = :excon

[Jesus]

Thanks for the good work! I'll review and merge as soon as I have a chance.

[Aupajo]

@Jesus Thanks for your suggestion! I've updated this PR to move middleware as a method on ConnectionBuilder.