Add API version to VCR cassetes

API version has been added to Facebook requests, so cassetes must be
updated.
Make FacebookAds::Client delegates to Koala

Our Client should delegates all its messages to Koala::Facebook::API.
It is also responsible to define the Facebook API version used by the
gem.

Note, we could make the Facebook API version configurable.
Add a top level client

Add a top level client, accessible through FacebookAds.client. You can
configure it via FacebookAds.configure as in the following example:

FacebookAds.configure do |config|
  config.access_token = 'MY_FACEBOOK_TOKEN'
end

The top level client is the default client used for all request made to
Facebook API from the gem whenever you do not provide a custom
configured client.
Merge pull request #4 from pitchtarget/feature/toplevel-client-config

Top level client