Support Delta GETs

[rlhotovy]

This adds support for the apps/delta endpoint. I also added an optional new constructor argument called eureka_host_name, meant to be similar to eureka_url, but is processed in the same way DNS instances are (using context and supporting eureka_port). Now you can do something like

ec = EurekaClient("my-app-name", eureka_host_name="my-server", eureka_port=8888, host_name="MY-PC")

and it will resolve self.eureka_urls = ["http://my-server:8888/eureka/v2/"], which feels a bit cleaner than having to set ec = EurekaClient(..., eureka_url="http://my-server:8888/eureka/v2/",...)