Implement retrying failed GET requests to Galaxy API calls

This functionality will be useful for our automated system.  We frequently experience failures where Galaxy fails to respond for a few seconds (actually, it responds with HTTP 200 but returns an empty response body). With this patch we can configure BioBlend to retry such failed GET requests so that upper layers never see the problem.

Rather, I haven't studied the BioBlend conventions regarding logging and exceptions.  Also, the decision of having the retry parameters configured as class variables of Client is debatable.  I've left the motivation in the comments. If you don't agree I'll happily discuss suggestions and modify the code.