Tests/support for block reference count

Add support for x-block-reference-count, according to the implementation in #121


Note: I was not sure how exactly to handle the scenarios covered by test_get_block_with_multiple_references
I understand that the test should only be trying to validate the one operation it is testing (Get Block while validating the reference count), but having a class/suite for each of the variations of the scenario seemed like a waste. Decided to add one setup step within the test itself (deleting one of the references) instead.
(Wish there was a way to do a data driven setUp of tests)