Change GoWorkerMixin.collect_metrics() to collect message metrics by default
Use LAST intead of MAX as the default aggregation method for worker metric collection
Update metric name construction for app workers to use the newly planned metric name convention
Fix vumi_app tests after metric name convention change
Add support for sqlite, memory and postgres databases.
Skip billing API tests on unsupported databases.
Fix syntax of postgres database creation line.
Fix postgres create user command.
Apparently only single quotes are allowed.
Make test environment variables available to all commands (e.g. django-admin.py syncdb).
Merge branch 'feature/issue-751-allow-running-tests-against-multiple-databases' into develop
Exclude new billing migrations from pep8. (@Hodgestar)
Merge pull request #748 from praekelt/feature/issue-748-change-default-worker-metric-aggregation-method-from-max-to-last

Change default worker metric aggregation method from MAX to LAST
Merge pull request #749 from praekelt/feature/issue-749-update-metric-name-construction-for-app-workers

Update metric name construction for app workers
Add js coverage output files to .gitignore (thanks @hodgestar, @jerith)
Change GoMetricsWorker to use the new way of specifying configs (involved using GoWorkerMixin and GoWorkerConfigMixin)
Mention that GoMetricsWorkerConfig's 'metrics_interval' is in seconds
Fix StreamingHTTPWorker metrics publishing test broken after vumi#640 (thanks @hodgestar)
Merge branch 'develop' into feature/issue-755-config-for-gometricsworker
Use worker_name instead of conversation_type when GoMetricsWorker publishes 'collect_metrics' commands