Feature/appdomain warmer

This does not so much fix the problem of the first hit cache building per appdomain but adds unit tests to proof and verify this behaviour.

Gave up the idea of adding a .Warmup() static method. Although it helped some it does not fully mitigate the problem.

See also:

http://stackoverflow.com/questions/24372525/cache-the-client-connection-with-elasticsearch-nest-so-the-first-call-to-the-cli

https://github.com/elasticsearch/elasticsearch-net/issues/742