elasticsearch: Ported requests connection class to requests_futures

* Based heavily off the requests connection class in elasticsearch-py;
  have added appropriate copyright notices.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>
tests: Created base ESLogTestCase unittest class

* Edited existing tests to use new abstract class.

Signed-off-by: Joe MacMahon <joe.macmahon@cern.ch>