Add oneshot support and dis-entangle MetricsManager from metrics.
Add tests for oneshot (and fix bugs revealed).
Pass the manager itself to Metric.manage so that a reference to the original manager is kept and more suitable errors can be reported. Add a .managed property for convenience and backwards compatibility. Remove a number of unnecessary calls to .manage in tests.
Update documentation and comments.
Merge branch 'feature/issue-640-oneshot-metrics' into develop
Merge branch 'develop' of github.com:praekelt/vumi into develop