Only show member's image if it exists

Related to https://github.com/openaustralia/publicwhip/issues/406

Introduces:
- [ ] a service class for checking if a resource exists
- [ ] modifications to `Person` to include methods that check if the resource associated to the large and small image URLs exist
- [ ] webmock for testing

The build has [been run up on travis](http://clicks.travis-ci.org/track/click/30007208/travis-ci.org?p=eyJzIjoiUlJJZG9ENG5HalNkZGhULXh0RE5IMVM4OHZrIiwidiI6MSwicCI6IntcInVcIjozMDAwNzIwOCxcInZcIjoxLFwidXJsXCI6XCJodHRwczpcXFwvXFxcL3RyYXZpcy1jaS5vcmdcXFwvbm92ZW1iZXJraWxvXFxcL3B1YmxpY3doaXBcXFwvYnVpbGRzXFxcLzM2MDQwMzcxXCIsXCJpZFwiOlwiMWY0YjgyZjM5MzI4NDRhNzlhNzEwZWYxYmZiODZhYjFcIixcInVybF9pZHNcIjpbXCI1Y2E3NmFjOTdkOWJjYWE2N2NiMTFmNGU4ZGI0N2NmNzk0ODQ5YTRiXCJdfSJ9) and is green