Rewrite to cleanly separate caching concern

[wojtha]

With my findings in last PR https://github.com/adamcooke/json-vat/pull/7 caching invalidation needs more complex handling so I've decided to do a significant rewrite to extract Configuration, PlainClient and CachedClient from the procedural code inside the JSONVAT module as the resolving of the cache invalidation would add another bunch of IFs.

At first I've tried composition, but it leads to me to really complex solution which felt like overengineering so I ended-up with old fashioned inheritance approach.

The code should be compatible with the original API. But I've decided to deprecate configuration writers since I've added configure and configuration methods.

Now you can also use the client classes directly

client = JSONVAT::CachedClient.new("my.host", MyCacheBackend.new)

JSONVAT.configure do |config| 
  config.client = JSONVAT::CachedClient.new("my.host", MyCacheBackend.new)
end

The code works I've run our in-app adapter tests against it successfully, but I feel it would be handy to add some tests or at least more documentation. So it should be considered as still in WIP.