Add build_web_site utility function to vumi.utils.
Use vumi.utils.build_web_site to shorten start_web_resource.
Make resource tree definition a dictionary.
Add test for build_web_site.
Add two more tests for build_web_site.
Add check that the overridden site-class isn't actually still LogFilterSite.
Merge branch 'feature/issue-539-factor-out-site-construction-to-vumi-utils' into develop