first pass at DNS discovery

* Doesn't handle failures gracefully
* Requires that you're in AWS
* Doesn't respect TTL's
correct comment
return the TTL of the region record for expiry purposes
add timer for DNS TTLs
only check DNS timeout if DNS discovery is enabled
compose serviceurls after discovering boxes via DNS
change the way that DNS TTLs are handled
Fix merge conflict caused by JSON changes
pull out repeated selection logic
Merge pull request #5 from hudl/discoverDNS

Allow the use of DNS discovery for eureka connections
Small tuning to the InstanceMetadata struct tags:
 - Raw needed a - for JSON
 - parsed doesn't anything since it is lowercase.
Merge branch 'master' of https://github.com/hudl/fargo