refactor galaxy client _get

This PR deals with a problem introduced in https://github.com/afgane/bioblend/pull/100. The method has been changed to return the request object instead of its (decoded) JSON content, but the check-errors-and-retry logic still deals with JSON problems (the part that catches `ValueError`, which cannot be triggered anymore).

As much as I hate to keep adding arguments to a method that already has plenty, I added a `json` arg that defaults to `True` (thus eliminating -- again :) -- the need for two separate methods) and reworked the logic so that it deals with JSON problems only when decoding is actually requested and is (hopefully) a bit more readable. The alternative would have been to move JSON-specific retry logic to the non-raw version of `_get`, making the code both redundant and more complex (the two methods would have needed to share the retry counter).